PRE-ASSESSMENT OF EVIDENTIAL VALUE OF EVIDENCES EXAMPLES FROM FORENSIC EXPERT PRACTICE

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1 by the Institute of Forensic Research ISSN PRE-ASSESSMENT OF EVIDENTIAL VALUE OF EVIDENCES EXAMPLES FROM FORENSIC EXPERT PRACTICE Paulina WOLAÑSKA-NOWAK De part ment of Ge net ics, In sti tute of Fo ren sic Sci ences, Kraków, Po land Abstract This pa per is based par tially on the ex pe ri ences of the au thor as an ex pert in fo ren sic ge net ics and par tially on ex pe ri ences gained dur ing her visit to Fo ren sic Sci ence Ser vice lab o ra to ries in Tri dent Court, Bir ming ham and the Lon don Met ro pol i tan Po lice Lab - o ra tory in June The au thor s dis cus sions and ex changes of ex pe ri ences in clined her to pres ent some is sues re lat ing to the co op er a tion of the Po lice and pub lic pros e cu tors with fo ren sic ex perts in Eng land and Wales. Fo ren sic Sci ence Ser vice has con - structed a framework for routine cooperation between Police and forensic experts called Case Assessment and Interpretation (CAI). A fun da men tal new fac tor pro posed by the CAI in the ap proach to ex am in ing ma te rial ev i dences from the scene of the crime is a pre cise es ti ma tion of the value of the in for ma tion that it is pos si ble to ob tain from pre lim i nary anal y sis of ev i dences even be fore be gin ning lab o ra tory anal y sis. In other words, the aim of CAI is to help the fact finder to make the right de ci sion. This en ables ra tio nal use of fi nan cial re sources for ex pen sive anal y ses (e.g. ge netic ones) and ra tio nal use of fo ren sic ex pert time, and may sig nif i cantly re duce the time of case re port prep a ra tion, which is very long in Po land now a days. Key words Ev i den tial value of ev i dence; DNA; Like li hood ra tio; Bayesian net works. Re ceived 29 Jan u ary 2010; ac cepted 31 March As sump tions of the CAI model In Eng land and Wales, be fore a case is sent to the pros e cu tor s of fice (the Crown Pros e cu tion Ser vice), where a de ci sion is made about the fu ture of the case on the ba sis of ap pro pri ate ar ti cles of the law, a pre-as - sess ment in ves ti ga tion is car ried out. (NB, the sit u a - tion is not uni form through out the United King dom: Scot land and North ern Ire land have sep a rate pros e cu - tion ser vices the COPFS and the Pub lic Pros e cu tion Ser vice re spec tively with some what dif fer ent re - mits). The pre-as sess ment in ves ti ga tion is per formed by the Po lice on the ba sis of an act from The Po - lice are in de pend ent from the Crown Pros e cu tion Ser - vice in terms of the scope of the in ves ti ga tion. Thus, the sit u a tion is dif fer ent from that in Po land (as well as most Eu ro pean le gal sys tems) in Eng land and Wales, the pros e cu tor does not have the right to give any guide lines to the Po lice with re gard to the di rec tion of the in ves ti ga tion and ac tions which should be car ried out in the in ves ti ga tion. How ever, the Po lice in Eng - land and Wales some times con sult the Crown Pros e cu - tion Ser vice about some things, but these mainly con cern the pos si bil ity of ad mis sion of ev i dence by a court and odds that the sus pect will be con victed. Most cases are not sent to the Crown Pros e cu tion Ser vice, be cause the po lice have the right to fin ish a case with out in volv ing ad di tional links in the chain of the ad min is tra tion of jus tice. When the Po lice de cide that a sus pect should be charged, then the case is sent to the Crown Pros e cu - tion Ser vice. The pros e cu tor anal y ses the case files, with a view to de cid ing whether the case should be

2 Pre-as sess ment of ev i den tial value of ev i dences ex am ples taken fur ther or dis con tin ued. The pros e cu tor should be guided by the Code for Crown Pros e cu tors in the de ci sion pro cess and should use two tests, i.e. the test of ev i dence and the test of pub lic in ter est when as sess - ing the case. If it is nec es sary to change the charges, which can also be mod i fied dur ing the course of a trial up to the mo ment when the court de cides to close the pro ceed ings or in sit u a tions where the case may be dis - missed (pro ceed ings can be dis missed up to the mo - ment when the case is pre sented be fore court), the pros e cu tor can con tact the Po lice, which can per form ad di tional in ves ti ga tions, if they de cide that they are jus ti fied. The ev i den tial test con cerns whether or not the ev i dence ma te rial gives a re al is tic pros pect of con vic tion and which cir cum stances sup port the sus - pect s ver sion. The men tioned pros pect is based on pre dict ing the be hav iour of the jury when the ev i dence is pre sented in court. This means that there is a greater prob a bil ity that the jury or bench of mag is trates, guided by the prin ci ples of law, will hand down a guilty ver - dict than not do so. The ev i den tial test also gives the pros e cu tor an op por tu nity to ob tain an swers to two ques tions. Firstly, can the col lected ev i dence be used in court? The prob lem of ad mis si bil ity of ev i dence is es pe cially im por tant. Un der the law in Eng land and Wales where there are stiff reg u la tions gov ern ing ev - i dence cer tain things can not con sti tute ev i dence and courts do not ad mit them. For ex am ple, the tes ti mony of a wit ness who was not pres ent at the scene of a crime and heard about the event from an other per son is one such piece of ev i dence so-called in di rect ev i - dence. Sec ondly, are the ev i dences cred i ble? On the is - sue of ev i dence cred i bil ity, the Code men tions sev eral cir cum stances which should be taken into ac count by the pros e cu tor, in clud ing the cred i bil ity of wit ness tes - ti mo nies. This re quires good co op er a tion with fo ren sic ex perts. If ev i dences col lected in the case do not ful fil re quire ments of the ev i den tial test then the case is dis - con tin ued [8]. A fun da men tal way in which a fo ren sic ex pert can help in an in ves ti ga tion is in ter pre ta tion of the ev i den - tial value of re sults of per formed anal y ses. Un doubt - edly, the most im por tant fea ture of fo ren sic sci ences is pro vid ing ad di tional in for ma tion on the ba sis of anal y - sis per formed in the lab o ra tory. In gen eral, an opin ion given by a fo ren sic ex pert is treated as a di rect de scrip - tion of re sults ob tained dur ing per formed anal y sis. There fore, the fo ren sic ex pert s con clu sions about the ev i den tial value of ana lysed ev i dences are con sid ered at the last stage of in ves ti ga tion or in the court room. This view has been thor oughly ana lysed in re cent years by Fo ren sic Sci ence Ser vice (FSS) un der the Case Assessment and Interpretation (CAI) project. The main aim of the CAI pro ject was to op ti mise costs which FSS of fers to its con sum ers. It as sumes a part - ner ship be tween the fact finder and the fo ren sic ex pert aimed at mak ing the most ef fec tive use of fi nan cial re - sources al lo cated to the ex pert opin ion. Such an ap - proach means that the use ful ness (ev i den tial value) of ana lysed ev i dences in ex plain ing all the dif fer ent pos - si ble as pects of crime cir cum stances is high lighted. This al lows a sen si ble di a log be tween the fact finder and the fo ren sic ex pert about the costs of an ex pert opin ion and about the con clu sions which can be drawn. More over, it makes it eas ier to make prop o si - tions (hy poth e ses) about the pos si bil ity of ob tain ing use ful ev i dence, hav ing heard the sus pect s ex pla na - tions and the vic tim s point of view. The CAI rec om - mends that in ter pre ta tion and eval u a tion of the ev i den tial value of ev i dence ma te rial should be the main sub ject of in ter est of the fo ren sic ex pert. If this ap proach is used, then, on the ba sis of case cir cum - stances, some con clu sions can be drawn even be fore per form ing anal y ses in the lab o ra tory. Ap pli ca tion of el e men tary sta tis ti cal meth ods plays an im por tant role in this ap proach. They are very use ful both at the in - ves ti ga tion stage, i.e. be fore anal y ses are per formed by the fo ren sic ex pert and in the eval u a tion of the ev i den - tial value of re sults of anal y sis ob tained in the fo ren sic laboratory [1]. 2. Pre-assessment of evidential value of evidence material The pro cess of eval u a tion of the ev i den tial value of an a lyt i cal re sults be gins at the mo ment when the fo - ren sic ex pert re ceives a com mis sion to ana lyse a par - tic u lar case. At this stage (the so-called pre-as sess ment stage), the fo ren sic ex pert be comes ac quainted with general information about the casework, questions submitted by the pros e cu tor, judge or po lice (fact finder) and the kind of de liv ered ev i dences. One of the first meth ods of sup port ing the pro cess of de ci sion-mak ing as to which types of anal y sis should be per formed, was de scribed by Cook et al [3]. It is based on a model which in cludes the like li hood ra tio as a mea sure of ev i - den tial value of an a lyt i cal re sults ob tained dur ing per - formed re search. The fi nal ver dict is con di tioned by many fac tors, in clud ing: in di cat ing the sus pect, his/her al ibi, the charges made against him/her, wit ness tes ti mo nies and their like li hoods, as well as the di ag nos tic value of ev i dences ana lysed in the case work. There fore, the prob a bil ity that the sus pect is guilty is a con di tional probability. In forensic practice, Bayes theorem is used

3 76 P. Wolañska-Nowak with the aim of cal cu lat ing the prob a bil ity of event G, that the sus pect is guilty, on con di tion that event E has oc curred (i.e. that the ev i dence, e.g. a DNA pro file found on the scene of the crime and one col lected from the sus pect match). In this sit u a tion, the sus pect s guilt is, broadly speak ing, the rea son for which the ev i dence was ob tained: P G E P G P ( E G ) ( ) ( ). {1} P( E) This prin ci ple al lows us to cal cu late the value of the con di tional prob a bil ity P( G E), i.e. the prob a bil ity that the sus pect is guilty if the ev i dence has been ob - tained. It is termed the prob a bil ity of guilt or a pos te ri - ori prob a bil ity, be cause it de scribes the prob a bil ity of the oc cur rence of the cause (G), if we know what ef fect it has caused (i.e. the de fined ev i dence E). As tran - spires from the above equa tion, its value de pends on the prob a bil ity of ob tain ing (find ing) ev i dence when it is as sumed that the sus pect is guilty: P( E G) and the a pri ori prob a bil ity that the sus pect is guilty (a sub jec - tive prob a bil ity): P(G) and also the prob a bil ity of ob - tain ing the ev i dence: P(E). There fore, the a pos te ri ori prob a bil ity that the sus pect is guilty, i.e. that the hy - poth e sis G is true, can be de ter mined on the ba sis of knowl edge about three fac tors: 1. a prio ri pro bab ili ty of event G P(G); 2. the pro bab ili ty of ob tai ning evi dence E, when the tru eness of hy pot hesis G is as sum ed P( E G); 3. pro bab ili ty of ob tai ning evi dence (in any po ssib le form) P(E). This law can be pre sented in the form of odds, which is more con ve nient from the prac ti cal point of view as it can be ex pressed ver bally more eas ily: P( G E) P( G) P( E G). {2} P( G E) P( G) P( E G) The ex pres sion on the left side of the equa tion is the odds of event G, i.e. that a sus pect is guilty, tak ing into ac count ob tained ev i dence (an a lyt i cal re sults) E. P( G) is the odds that a sus pect is guilty (G) tak ing P( G) into ac count in for ma tion ob tained dur ing an in ves ti ga - tion, wit ness tes ti mo nies etc., which are in de pend ent of ev i dence (E). There fore it is the odds that the sus - pect is guilty a pri ori. The sec ond part of the equa tion, i.e. the ex pres sion on the right, is the so-called like li - hood ra tio (LR) for ob tain ing ev i dence E when two al - ter na tive hy poth e ses are as sumed: G and G. The G hy poth e sis is the equiv a lent of the pros e cu tion hy poth - e sis, which is most fre quently de scribed as H p. The notg hy poth e sis is the equiv a lent of the de fence hy - poth e sis, which is fre quently de scribed as H d. In other words, the like li hood ra tio is the ra tio of two con di - tional prob a bil i ties: the oc cur rence of ev i dence if the sus pect is guilty and the oc cur rence of ev i dence if the sus pect is not guilty, i.e. it oc curred by chance. It should be men tioned that the like li hood ra tio is not a prob a bil ity and its val ues can be as tro nom i cally high. The view that the like li hood ra tio, in the sense of the Bayesian the o rem, is the most re li able and log i cal method of pre sent ing the ev i den tial value of ev i dence in the court room has be come wide spread. Each value LR > 1 sup ports the pros e cu tion hy poth e sis H p, and each value LR < 1 sup ports the de fence hy poth e sis. The strength of such sup port de pends on the LR value. The like li hood ra tio is a key fea ture in the in ter pre - ta tion of the fi nal ev i dence value (af ter per form ing of anal y sis in the lab o ra tory). How ever, it can also be used within the CAI model to give an ex cel lent pre lim - i nary value of de liv ered ev i dence (be fore anal y sis has been performed). The pro posed method of pro ceed ing is aimed at in - creas ing the ef fec tive ness of the work of the fo ren sic ex pert in col lab o ra tion with the fact finder. An other aim is to en able the fact finder to make the op ti mal de - ci sion. This does not mean that in ter pre ta tion of the ob tained re sults is an el e ment of the sub jec tive ap - proach of the fo ren sic ex pert. No doubt, pure in tu ition plays a part in se lec tion of the or der of per formed ex - am i na tions, which then ini ti ates ra tio nal anal y sis. The ex pe ri ence of the fact finder is, of course, very im por - tant dur ing se lec tion of ev i dences which are sent for anal y sis to the fo ren sic lab o ra tory; how ever, as far as pos si ble, pure prag ma tism should pre dom i nate. The ex am ple be low il lus trates the ap proach in the pro cess of decision making concerning the necessity of studying par tic u lar ev i dences se cured in a sex ual as sault case. The cir cum stances of the case (crime) pre sented in the files sent by the fact finder were as fol lows: prop o - si tion H p the woman stated that she was raped at her friend s home (full pen e tra tion oc curred); prop o si tion H d the sus pect main tained that the woman vis ited him willingly; they only pet ted while re main ing clothed; he ejaculated into his own un der wear. Af ter re ceiv ing the above in for ma tion, fo ren sic ex perts can es ti mate the conditional probabilities of various events, depending on pro vi sion ally as sumed hy poth e ses ex plain ing the course of the event, tak ing into ac count in for ma - tion de liv ered by the fact finder (on the right side of the ver ti cal line ( given ) are con di tion ally ana lysed hy - poth e ses:

4 Pre-as sess ment of ev i den tial value of ev i dences ex am ples Pr(de term ina tion of lar ge amou nt of sperm in va - gina pro pos iti on H p ) = 0.9; 2. Pr(de term ina tion of lar ge amou nt of sperm in va - gina pro pos iti on H d ) = ; 3. Pr(de term ina tion of tra ce amou nt of sperm in va - gina pro pos iti on H p ) = 0.09; 4. Pr(de term ina tion of tra ce amou nt of sperm in va - gina pro pos iti on H d ) = ; 5. Pr(a lack of sperm in va gina proposition H p ) = 0.01; 6. Pr(a lack of sperm in va gina proposition H d ) = Fur ther, in an an a log i cal way, other prob a bil i ties can be es ti mated on the ba sis of sub jec tive fo ren sic ex - pert knowledge): 7. Pr(de term ina tion of lar ge amou nt of sperm in side su spect s un der we ar pro pos iti on H p ) = 0.001; 8. Pr(de term ina tion of lar ge amou nt of sperm in side su spect s un der we ar pro pos iti on H d ) = 0.99; 9. Pr(de term ina tion of tra ces of sperm in side su - spect s un der we ar pro pos iti on H p ) = 0.01; 10. Pr(de term ina tion of tra ces of sperm in side su - spect s un der we ar pro pos iti on H d ) = 0.001; 11. Pr(a lack of sperm in side su spect s un der we ar pro - position H p ) = 0.99; 12. Pr(a lack of sperm in side su spect s un der we ar pro - position H d ) = Val ues of like li hood ra tio LR can be cal cu lated for var i ous com bi na tions of con sid ered prop o si tions (hy - poth e ses) H p and H d in the ana lysed case af ter es ti - ma tion of the above con di tional probabilities: obtaining evidenceh p istrue LR Pr ( ) Pr( obtaining evidenceh d istrue), {3} Therefore: LR 1/2 = 0.9/ = 9000; LR 3/4 = 0.09/ = 100; LR 5/6 = 0.01/ ; LR 7/8 = 0.001/0.99 = 0.001; LR 9/10 = 0.01/0.001 = 10; LR 11/12 = 0.99/0.001 = 990. Anal y sis of the ob tained re sults can pro vide an in - di ca tion to the fact finder as to which ev i dences should be ana lysed. What con clu sions can be drawn on the ba sis of these re sults? The larg est like li hood ra tio value can be ob tained when a swab from a va gina is ana lysed be cause it al lows us to ob tain a value of LR 1/2 = 0.9/ = 9000 af ter per form ing anal y sis. The value of this LR means that it could be ex pected that de ter mi na tion of a large amount of sperm in the va gina of the woman is ca more likely when hy poth e sis H p is true (prosecution hypothesis) than determination of a large amount of sperm in the va gina of the woman when hy poth e sis H d is true (de fence hy poth e sis). Such high ev i den tial value of the ob tained ev i dence would strongly sup port the pros e cu tion hy poth e sis (H p ). In turn, if sperm were not de ter mined in the va gina of the woman, then LR 5/6 = 0.01/ would strongly sup port the de fence hy poth e sis, i.e. H d. Af ter performance of the preliminary calculations presented above, the fo ren sic ex pert could sug gest to the fact finder that ad di tional anal y sis of the sus pect s un der wear should not be car ried out since such ev i dence would not pro - vide any ad di tional in for ma tion that is nec es sary to es - tab lish the cir cum stances of the event, but at the same time it would sig nif i cantly in crease the cost of the ex - pert opin ion. The pre sented ap proach is sig nif i cantly dif fer ent from that adopted by Pol ish fo ren sic ex perts. Fact find ers fre quently com mis sion anal y sis of a few dozen ev i dences col lected on the crime scene and from vic - tims and sus pects. Here is an ex am ple from a case of this kind: case files in di cated that that vic tim and the sus pect had drunk al co hol non-stop for three days, as at tested to by al most all the lo cal com mu nity. At the end, one of the rev el lers grabbed hold of an axe, whilst the sec ond grasped a knife. The one who grabbed the axe was faster and killed his col league. All the ev i - dences (traces) col lected at the scene of the crime, in - clud ing 60 cig a rette stubs, were sent for ge netic anal y sis with the aim of es tab lish ing whether the sus pect was or was not on the scene of the crime! Items of cloth ing col lected from the de ceased (in clud ing un der wear) which were to tally soaked in blood were also sent for anal y sis. What kind of use ful in for ma tion could be ob - tained from anal y sis of such ev i dence ma te rial? The fol low ing is a pre lim i nary anal y sis of the pos - si bil ity of find ing traces orig i nat ing from the as sail ant on the de ceased s clothes: hy poth e sis H p the sus pect caused the death of the vic tim us ing an axe; hy poth e sis H d : a per son other than the sus pect caused the death of the vic tim us ing an axe. The fol low ing prob a bil i ties can be es ti mated on the ba sis of knowl edge about the con di tion of the de ceased s clothes, which were to tally soaked by blood (it should be men tioned that most prob a bly, it was the vic tim s blood, which was also in - di cated by re sults of post mor tem ex am i na tions car ried out each time be fore sending these evidences to the forensic laboratory): Pr(traces orig i nat ing from sus pect are found hy - poth e sis H p ) = ; Pr(traces orig i nat ing from sus pect are found hy - poth e sis H d ) = The like li hood ra tio ob tained on the ba sis of these as sumed prob a bil i ties was: LR = / = 1.

5 78 P. Wolañska-Nowak The ob tained neu tral value of LR = 1 means that anal y sis of the men tioned ev i dences (the to tally bloodsoaked clothes of the de ceased) will not sup port ei ther the pros e cu tion or the de fence hy poth e sis and there fore the huge costs of per form ing such anal y ses will be wasted. The fo ren sic ex pert, on the ba sis of per - sonal ex pe ri ence, could sug gest to the fact finder that anal y sis of such ev i dence ma te rial will not pro vide any use ful ad di tional in for ma tion which could help to solve the case. On the ba sis of tes ti mo nies of nu mer ous wit nesses of the par ty ing, which con firmed the fre quent and long-last ing vis its of the sus pect at the vic tim s home and also knowl edge about ha bit ual cig a rette smok ing by both men, the fol low ing val ues of con di tional prob - a bil i ties can be estimated (predicted): Pr(pres ence of sus pect s cig a rette butts at vic tim s home hy poth e sis H p ) = 0.99; Pr(pres ence of sus pect s cig a rette butts at vic tim s home hy poth e sis H d ) = Again, in this case, LR = 1 means that the ev i den tial value of such ev i dence ma te rial is neu tral and does not sup port any of the con sid ered hy poth e ses and, fur ther - more, very ex pen sive anal y sis of a few dozen cig a rette butts will not be help ful in ex plain ing the case cir cum - stances. How ever, a de ci sion about anal y sis of the sus - pect s clothes could pro vide im por tant and use ful information about the cir cum stances of the event. Let us eval u ate the fol low ing con di tional prob a bil i ties: Pr(pres ence of vic tim s blood on sus pect s clothes hy - pothesis H p ) = 0.98, Pr(pres ence of vic tim s blood on sus pect s clothes hy - pothesis H d )= 0.02 (e.g. the sus pect could have given first aid to the vic tim). In this ex am ple, LR = 0.98/0.02 = 49, which means that find ing of the de ceased s blood on the sus pect s clothes is 49 times more likely if the pros e cu tion hy - poth e sis is true (sus pect is per pe tra tor hy poth e sis H p ) than if blood is found when the de fence hy poth e sis is true (sus pect is not guilty hy poth e sis H d in the ana - lysed case). As could be ex pected, pre-as sess ment of the ev i - den tial value of the sus pect s clothes al lowed us to con clude that such ma te rial should be sent for anal y sis to a fo ren sic laboratory. 3. Ap pli ca tion of Bayesian Net works to pre-as sess ment of ev i den tial value of evidence material A Bayesian Net work is a graph i cal model which pres ents de pend en cies be tween vari ables. The nodes in a net work rep re sent vari ables such as pres ence of sperm in va gina, and can have var i ous states. In the case de scribed above, they are: a lot of sperm (a lot of), i.e. an amount which could eas ily be de ter mined by ap pli ca tion of pre lim i nary, non-ge netic tests, which are based on de tec tion of fac tors that are char ac ter is tic for sperm. Other states are: trace amount of sperm (trace) where re sults of per formed tests give weak pos i tive an swers or non-con clu sive ones and lack of sperm (no) in cases where tests do not de tect sperm. Cer tain con di tional prob a bil ity val ues can be as cribed to each of these states, which de pend on knowl edge about the true ness of the con sid ered hy poth e sis. The as sumed val ues de pend on fo ren sic ex pert knowl edge (which is par tially sub jec tive but is also based on re - sults of pre vi ous case work). Vari ables which do not have so-called par ent nodes i.e. there is a lack of node(s) which could have an in flu ence on the prob a - bil i ties of their states have states with as sumed a pri - ori prob a bil i ties (non-con di tional ones). For ex am ple, the node fa ther hood in Fig ure 1 has two states: yes and no. Ar rows in di cate de pend en cies be tween vari - ables. In Bayesian Net works, de pend en cies be tween vari ables and ar rows as sume the form of an acy clic graph, i.e. there is a lack of loops in such a graph. A fun - da men tal idea of graph i cal meth ods is to construct them from sim ple parts (ba sic con nec tions). Also, the men tioned pre-as sess ment es ti ma tion of the pos si bil i - ties of ob tain ing in for ma tion from ana lysed ma te ri als can be il lus trated in a graph i cal way by ap pli ca tion of Bayesian Net works mod els [10], since they al low visu ali sa tion of the es tab lished method of rea son ing. When there are two al ter na tive hy poth e ses of aris ing of ev i dence, then the nec es sary con di tional prob a bil i - ties can be cal cu lated in a rel a tively easy way. But, in the case of con sid er ation of more than two hy poth e ses es tab lish ing and es ti mat ing the de pend en cies be tween them is much more com pli cated. Let us ana lyse the fol low ing ex am ple of a pa ter nity in ves ti ga tion: sam - ples from the child (male) and brother of the sus pect, whose place of res i dence could not be es tab lished (Fig ure 1) were pro vided as ev i dence ma te ri als. The node re lat ing to the pa ter nity hy poth e sis has two states: yes and no with a pri ori prob a bil i ties equal to 0.5, which means that there is a lack of any data on re la tions be tween the sus pect and the child s mother. Identifiler and Yfiler nodes rep re sent pos si ble re sults of ge netic anal y sis car ried out by ap pli ca tion of the men tioned kits. The Identifiler kit (Applera) al lows us to de ter mine the DNA pro file within the range of six - teen so-called autosomal STR mark ers. How ever, ap - pli ca tion of the Yfiler kit (Applera) al lows us to de termine the haplotype of the Y chro mo some, which has

6 Pre-as sess ment of ev i den tial value of ev i dences ex am ples been in her ited from fa ther to son. Prob a bil i ties of ob - tain ing ap prox i mate re sults of anal y sis are pre sented in ta bles of con di tional prob a bil i ties (Ta ble I) as sumed for each vari able, and they are also pre sented in Fig - ure 1 in boxes at tached to each node. TA BLE I. CONDITIONAL PROBABILITIES FOR INFORMATION THAT IT IS POSSIBLE TO OBTAIN IN THE CASE OF A DISPUTED PATERNITY INVESTIGATION Ge netic test (M) Pr(M H p ) Pr(M H d ) LR Identifiler Yfiler Sam ples col lected from the sus pect s child (male) and brother were used as con trol ma te rial. The paternity node has two states de scribed in the ta ble as: H p the sus pect is the fa ther of the child, H d the sus pect is not the fa ther of the child. A pri ori probabilities for hypothesis H p and H d were as sumed to be equal to 0.5. M type of kit ap plied for anal y sis; Identifiler al lows de ter mi na tion of DNA pro file within the range of autosomal mark ers, Yfiler al lows de ter mi na tion of the haplotype within the range of the Y chro mo some in her ited from fa ther to son. Possible values of the likelihood ratio (LR) are ra tios of con di - tional LR = Pr(M H p )/Pr(M H d ). Fig. 1. An ex am ple of a Bayesian Net work which il lus trates a case of a dis puted pa ter nity in ves ti ga tion in a sit u a tion where sam ples from the child (male) and brother of the sus - pect were de liv ered for anal y sis. The pa ter nity hy poth e sis node has two states: yes and no with a pri ori prob a bil i - ties equal to 0.5. Identifiler and Yfiler nodes rep re sent re sults of genetic analysis that are pos si ble to achieve by ap pli ca tion of the men tioned kits. When it is as sumed that the sus pect is the bi o log i - cal fa ther of the child, and his brother in her its the haplotype of the Y chro mo some from their fa ther, then the child should have the same haplotype of the Y chro - mosome (the pos si bil ity of oc cur rence of mu ta tion during spermatogenesis is not taken into ac count here). There fore, when a match within the range of the Yfiler kit is found, then it can de liver much more im por tant in for ma tion than an anal y sis of autosomal mark ers. When a match is not de ter mined, then the hy poth e sis that the ex am ined per sons are re lated can be re jected, and then per for mance of ex pen sive anal y sis by ap pli - ca tion of autosomal mark ers is not nec es sary. Ex pe ri - ence al lows us to con clude that anal y sis of autosomal mark ers will not de liver much in for ma tion that is im - por tant for draw ing con clu sions in the pre sented case. This gives an in di ca tion to fo ren sic ex perts that an anal y sis of the type of Y chro mo some should be car - ried out first. An other ex am ple il lus trat ing the prob lem of preas sess ment of ev i den tial value of ana lysed ev i dence ma te rial by ap pli ca tion of Bayesian Net works mod els is pre sented in Fig ure 2. It il lus trates the sit u a tion de - scribed in sec tion 2 of this pa per. The node H de scribes a hypothesis concerning the circumstances of the event: H p pros e cu tion hy poth e sis, i.e. the woman was raped at a friend s house (there was full pen e tra tion); H d de fence hy poth e sis, the woman vis ited a friend s house and only ca ress ing in clothes oc curred and the sus pect only ejac u lated into his un der wear. Node H has only two pos si ble states: yes and no with equal a pri ori prob a bil i ties, i.e. 0.5 each. This means that there is a lack of ad di tional in for ma tion in di cat ing which of

7 80 P. Wolañska-Nowak amount of sperm in sus pect s un der wear. It was men - tioned ear lier that these vari ables can have three states: a lot of sperm (a lot of), where there is an amount which can eas ily be de tected by ap pli ca tion of pre lim i - nary tests and non-ge netic ones based on de ter mi na - tion of fac tors characteric for sperm; trace amount (trace) where re sults of per formed tests give weak pos i tive an swers or non-con clu sive ones and lack of sperm (no) where sperm is not de tected by all ap - plied meth ods. Some con di tional prob a bil i ties could be as cribed to each state de pend ing on ac cepted knowl - edge as to whether hy poth e sis H is true or not. Ta ble II pres ents val ues of con di tional prob a bil i ties of ana - lysed vari ables and val ues of like li hood ra tio (LR) which could be ob tained. TA BLE II. TABLE OF CONDITIONAL PROBABILITIES FOR CONSIDERED VARIABLES H Amount of material Pr(V H p ) Pr(V H d ) LR V A lot of Trace No Pr(T H p ) Pr(T H d ) LR T A lot of ,001 Trace No Fig. 2. An ex am ple of pre-as sess ment of the ev i den tial value of ev i dence ma te rial in the case where the vic tim s ver sion is con sid ered. H p : the woman claims that she was raped in a friend s house (there was full pen e tra tion) and the sus - pect s ver sion, H d : the woman vis ited his house and only ca - ress ing in clothes oc curred and the sus pect only ejac u lated in his un der wear. Nodes V and T rep re sent pos si ble re sults of per formed anal y sis: V amount of sperm in vic tim s va gina, T amount of sperm in sus pect s un der wear. the prop o si tions that put for ward by the vic tim or the sus pect is more prob a ble. More over, nodes V and T rep re sent the fol low ing vari ables: V amount of sperm in vic tim s va gina, T Pr(V H p ) conditional probability of obtaining a certain amount of sperm in the woman s va gina un der the as sump tion that the prosecution hypothesis is correct; Pr(V H d ) con di tional prob - a bil ity of ob tain ing a cer tain amount of sperm in the woman s va gina un der the as sump tion that the de fence hy poth e sis is cor - rect, Pr(T H p ) conditional probability of obtaining a certain amount of sperm in the sus pect s un der wear un der the as sump - tion that the prosecution hypothesis is correct; Pr(T H d ) con - di tional prob a bil ity of ob tain ing a cer tain amount of sperm in the sus pect s un der wear un der the as sump tion that the de fence hypothesis is correct, LR = Pr(V H p )/Pr(V H d ) or re spec tively: LR = Pr(T H p )/Pr(T H d ). Fig ure 2 shows that en ter ing the in for ma tion that lots of sperm has been found in the vic tim s va gina into the net work gives al most com plete cer tainty that the vic tim s ver sion is true, i.e. she was raped. How - ever, when the ev i dence con sists in lots of sperm be ing found in the sus pect s un der wear, then the vic tim s ver sion is very im prob a ble. In the ad di tional net work, the pos si bil ity that the sus pect s ver sion H d is true is pre sented in this case, de tec tion of a large amount of sperm in the sus pect s un der wear is very prob a ble.

8 Pre-as sess ment of ev i den tial value of ev i dences ex am ples Val ues of LR which could pos si bly be ob tained al low us to con clude about the use ful ness of anal y sis of par - tic u lar ev i dence ma te ri als. From Ta ble II, it tran spires that the most in for ma tive re sult would be de tec tion of a large amount of sperm in the vic tim s va gina. There - fore, it is not use ful to ana lyse both ev i dences, as anal - y sis of only one ev i dence allows us to obtain useful information about the circumstances of the event. Cal cu la tion of like li hood ra tios for par tic u lar pairs of hy poth e ses is very easy it is enough to di vide one of the prob a bil ity val ues by the other, e.g. or Pr( T1 3 H p ) LR Pr( T H ) 1 3 Pr( V1 3 H p ) LR Pr( V H ) 1 3 d d {4} {5} de pend ing on the re quest(s) sent by fact find ers to the fo ren sic ex pert. When mak ing a de ci sion about the num ber and or - der of anal y ses of ev i dence ma te ri als in a par tic u lar case, it is nec es sary to have at least a gen eral knowl - edge about the cir cum stances of the crime (event), such as time, place, ac tiv i ties, tes ti mo nies of wit nesses etc. The en tirety of this in for ma tion con sti tutes an out - line of cir cum stances which have an in flu ence on the de ci sion made by the fact finder as to which ev i dences should be sent for anal y sis and what kind of an swers should be pro vided by the fo ren sic ex pert. It is very im por tant to em pha size that this out line will have el e - ments of un cer tainty, and un con vinc ing or even false frag ments. It should be men tioned that this scheme will change, per haps many times, from the be gin ning of the work on the case up to the judge s ver dict. The fo ren sic ex pert car ries out re search, ex am i na tions and anal y ses which pro vide in for ma tion within this frame - work and in ac cor dance with the rec om men da tions of the fact finder. There fore, the aim of the fo ren sic ex - pert is to eval u ate the evidential value of obtained results using well established scientific methods. The Bayesian ap proach al lows us to ap ply prob a - bil ity the ory to draw ing con clu sions un der con di tions of un cer tainty, i.e. when there is a lack of cer tain in for - ma tion. It takes into ac count the fun da men tal im por - tance of the like li hood ra tio, which ful fils three main prin ci ples of in ter pre ta tion of sci en tific evidence for forensic purposes: 1. in terp reta tion of scien tif ic evi dence is made wi thin the fra mew ork of the cir cums tances of the anal - ysed case, 2. in terp reta tion is only cre dib le when two or more opposite propositions or hypotheses are considered re lat ing to the co urse of the cri me, re sult ing from test im oni es of both the su spect and the pro sec uti on; 3. the role of the fo rens ic expert is to anal yse the pro - bab ili ty of evi dence ob ser vati on (occur rence) as - sum ing hy pot heses con sid ered earl ier [4]. The CAI model as sumes that its rules and as sump - tions should be the stan dard, ir re spec tive of the com - plex ity of the case. Their ap pli ca tion in prac tice en sures that a log i cal eval u a tion can be carried out. In ter pre ta tion of re sults con sti tutes the fi nal part of eval u a tion of ev i dences in case work. How ever, un der the CAI model, it is as sumed that such in ter pre ta tion is not ad vis able in iso la tion from gen eral knowl edge about the case cir cum stances. It was men tioned ear lier that the fo ren sic ex pert should have the pos si bil ity of car ry ing out a pre-as sess ment of de liv ered ev i dence ma te rial. At this stage, the fo ren sic ex pert be gins to set the ques tions which he or she will have to an swer in the case re port (ex pert opin ion) or in court. To put it more for mally, the fo ren sic ex pert should take part in the pro cess of for mu lat ing hy poth e ses as to the case cir cum stances and at the same time be aware of the ev - i den tial value that can be ex pected (af ter full anal y sis). This leads to the ne ces sity of in for ma tion ex change be tween the fact finder and the forensic expert before there is any expenditure on examinations. 4. A hierarchy of considered hypotheses The CAI model as sumes that the fo ren sic ex pert can not con cen trate on only one hy poth e sis when eval - u a tion of the ev i den tial value of ob tained an a lyt i cal re - sults ma te rial is per formed [3]. The ob tained re sults should be ana lysed tak ing into ac count at least two prop o si tions. Such a con tra dic tory sys tem re flects the prop o si tions of pros e cu tion and de fence. Of course, cir cum stances of a case can change dur ing a trial. There fore, it is very im por tant that the fo ren sic ex pert has as thor ough a look at the case as pos si ble and takes into ac count var i ous pos si ble hy poth e ses (cir cum - stances). In gen eral, the hi er ar chy of prop o si tions has three lev els, al though a cat e gor i cal dis tin guish ing be - tween var i ous lev els can some times be dif fi cult as in some cases the border between considered levels can even fade. These levels are related to: 1 the so urce of the evi dence ma ter ial (so urce level); 2 the act ivity of the su spect (act ivity level); 3 the gu ilt of the su spect (offen ce level). The high est, third level, con cerns the es tab lished guilt of the sus pect and only the judge (court) can eval -

9 82 P. Wolañska-Nowak u ate whether the con sid ered hy poth e ses are true or false, e.g. H p Mr. X raped Mrs. V and its al ter na tive, i.e. H d an other man raped Mrs. V. On the ba sis of the per formed anal y sis, the fo ren sic ex pert can only give an an swer to a ques tion re lated to the sec ond level, i.e. the ac tiv ity level, e.g. H p Mr. X had sex with Mrs. V; H d an other man had sex with Mrs. V. When the fo - ren sic ex pert wishes to give an an swer to ques tions con cern ing this level, then the case cir cum stances should be taken into ac count. More over, at this stage, the pos - si bil ity of trans fer of bi o log i cal ma te rial should be taken into ac count. How ever, the fo ren sic ex pert, in most cases, does not have in for ma tion which would al - low him/her to draw con clu sions con cern ing the activity level, and re sults of anal y sis per formed in a fo ren sic lab o ra tory mainly con cern the first level, i.e. prop o si - tions re lat ing to the source of the bi o log i cal ma te rial, e.g.: H p sperm orig i nates from Mr. X, H d sperm orig i nates from an other man. With the rapid de vel op ment of DNA pro fil ing tech - niques, es pe cially re lat ing to the sen si tiv ity of an a lyt i - cal meth ods, a new im por tant prob lem is emerg ing. Fo ren sic ex perts should be con scious of the dif fer ence be tween for mal prop o si tions which arise, at least partly, de pend ing on the cir cum stances of the case, and less for mal ways of ex plain ing them. If the fo ren - sic ex pert con sid ers that it is more prob a ble that par tic - u lar ev i dence will be ob tained given the true ness of the pros e cu tion hy poth e sis than given the de fence hy poth - e sis, then this sup ports the pros e cu tion hy poth e sis. On the other hand, if ob tain ing of the ev i dence is more prob a ble if the de fence hy poth e sis is true, then such ev i dence is more likely to sup port the de fence strat egy. In both cases, the ev i den tial value de pends on the value of the like li hood ra tio. This con cep tion is the same for any kind of ev i dence ma te rial and the ob - tained value of the like li hood ra tio can some times be ex pressed in ver bal form [2, 12]. In con clu sion, it should be em pha sized that the fo ren sic ex pert should ob tain in for ma tion about the event (crime) cir cum - stances from the fact finder even be fore be gin ning anal y sis as this al lows: for mu la tion of hy poth e ses which the fo ren sic ex - pert has to eval u ate; if cer tain hy poth e ses are as sumed to be true, then some as pects of case cir cum stances al low the ex - pert to con clude about the prob a bil ity of ob tain ing ev i dence. In for ma tion about the time of the event and the sus pect s ac tiv ity al low the ex pert to con - sider the pos si bil ity of trans fer and per sis tence of ev i dence, which could be in de pend ent of the time of the crime. In cases where re sults of DNA pro fil - ing are con sid ered as ev i dence, a phys i cal de scrip - tion of the per pe tra tor could help the ex pert to make a de ci sion as to which da ta base with in for ma - tion about fre quency of oc cur rence of par tic u lar types of al leles and haplotypes should be used with the aim of de ter mi na tion of fre quency of oc cur - rence of the par tic u lar DNA pro file in the gen eral pop u la tion; other as pects of case cir cum stances can have an in - flu ence on eval u a tion of a pri ori prob a bil i ties for a par tic u lar hy poth e sis. For ex am ple, in cases con - cern ing dis puted pa ter nity, e.g. in volv ing in cest, other as pects could be wit ness tes ti mo nies or in for - ma tion about living together. The role of the fact finder is to es tab lish which in - for ma tion could be im por tant for the fo ren sic ex pert. An out line of the cir cum stances of a case is not just a set of facts. It is im por tant for the fo ren sic ex pert to have ap pro pri ate in for ma tion which al lows him/her to ex plain (eval u ate) the re sults of his/her anal y sis. It is also im por tant for the fo ren sic ex pert to ob tain in for - ma tion about changes con cern ing the case cir cum - stances as they could be im por tant in ver i fi ca tion of previously assumed hypotheses. 5. Con clu sions The model of pre-as sess ment of ev i den tial value of ev i dence ma te rial which has been worked out by fo - ren sic sci en tists from the Fo ren sic Sci ence Ser vice, is a prag matic ap proach to es ti ma tion of the ra tio of costs to in for ma tion which it is pos si ble to ob tain af ter se - lected ev i dence anal y sis. Aitken [1] sum ma rised the pre-as sess ment process in a few points: col lec tion of in for ma tion about case cir cum stances which could be im por tant for the fo ren sic ex pert; for mu la tion of ques tions, to which the fo ren sic ex - pert should give an swers and prop o si tion of hy - poth e ses which the ex pert can con sider with the aim of eval u at ing the ev i den tial value of the ana - lysed ma te rial; de ter mi na tion of fea tures which should be con sid - ered in the es ti ma tion of the ap prox i mate value of the like li hood ra tio for the pro posed hy poth e ses; eval u a tion of the ex pected value of the like li hood ra tio which can be ob tained on the ba sis of in for - ma tion about case cir cum stances; de ter mi na tion of strat egy of per formed anal y ses. Aitken also em pha sized that re sults of pre-as sess - ment anal y sis are a part of the pro cess of eval u a tion of the ev i den tial value of ma te rial, but the pre-as sess ment anal y sis in it self can not be used as ev i dence in court. For ex am ple, in di cat ing the sus pect on the ba sis of an

10 Pre-as sess ment of ev i den tial value of ev i dences ex am ples ob served match be tween the DNA pro file of the sus - pect and the DNA pro file of a rel a tive of the sus pect, whose pro file is pres ent in a da ta base could help to iden tify the sus pect but the re sult of such a test is not ev i dence. Nev er the less, it can seriously help in an investigation. References 1. Aitken C. G. G., Sta tis tics in fo ren sic Sci ence. Part I. An aid to in ves ti ga tion, Prob lems of Fo ren sic Sci ences 2006, 65, Branicki W., Kupiec T., Wolañska-Nowak P., Badania DNA dla celów s¹dowych, Wydawnictwo Instytutu Ekspertyz S¹dowych, Kraków Cook R., Evett I. W., Jack son G. [et al.], A model for case as sess ment and in ter pre ta tion, Sci ence & Jus tice 1998, 38, De For est P. R., Re cap tur ing the es sence of criminalistics, Sci ence & Jus tice 1999, 39, Evett I. W., Avoid ing the trans posed con di tional, Sci ence & Jus tice 1995, 35, Evett I. W., Jack son G., Lam bert J. A., More on the hi er ar - chy of prop o si tions: ex plor ing the dis tinc tion be tween the ex pla na tion and prop o si tions, Sci ence & Jus tice 2000, 40, Evett I. W., Jack son G., Lam bert J. A. [et al.], The im pact of the prin ci ples of ev i dence in ter pre ta tion on the struc - ture and con tent of state ments, Sci ence & Jus tice 2000, 40, Michalczuk C., Struktura i funkcjonowanie prokuratury w Wielkiej Brytanii, Prokuratura i Prawo 2003, 11, Robertson B., Vignaux G. A., Interpreting evidence. Eval - uating Forensic Science in the Courtroom, John Wiley & Sons, Chichester Taroni F., Aitken C. G. G., Garbolino P. [et al.], Bayesian Net works and proba bil is tic in fer ence in fo ren sic science, John Wiley & Sons, Chichester Wolañska-Nowak P., Branicki W., Baza danych profili DNA nowe narzêdzie dla wymiaru sprawiedliwoœci, Prokuratura i Prawo 2000, 5, Wolañska-Nowak P., Branicki W., Interpretacja wyników badania DNA w procesie karnym, Prokuratura i Prawo 2003, 11, Cor re spond ing au thor Paulina Wolañska-Nowak Instytut Ekspertyz S¹dowych ul. Westerplatte 9 PL Kraków pwolanska@ies.krakow.pl

11 WSTÊPNE OSZACOWANIE WARTOŒCI DOWODOWEJ PRZYK ADY Z PRAKTYKI BIEG EGO S DOWEGO 1. Za³o enia mo delu CAI W Wielkiej Brytanii zanim sprawa trafi do prokuratu - ry, gdzie w oparc iu o za sady ko deksu za dec ydu je siê o dal szych jej lo sach, prze prow adza siê post êpow anie przygotowawcze. Prowadzi je wy³¹cznie policja na mocy ustawy z 1984 roku. W za kres ie pro wad zone go œle dzt wa policja jest niezale na od prokuratury. Inaczej ni w Pol - sce (oraz w przy padku wiêk szoœ ci syst emów europej - skich) pro kur ator nie ma pra wa wy daw ania po lic ji ja - kich kolw iek wy tyczn ych od noœnie do kie runku pos tê po - wania oraz wy kon ywa nia cz ynn oœci. Mimo i po lic ja bry tyjs ka kon sult uje nie kiedy z pro kur atur¹ pew ne posu - niêcia, to do tycz¹ one ra czej mo liwoœci do puszc zenia do wodów przez s¹d oraz oceny szans na ska zan ie spraw - cy. Wiêks zoœæ spraw nie tra fia w ogó le do pro kur atu ry, gdy po lic ja ma mo li woœæ za koñ czenia ich bez po trzeby an ga ow ania w to do datk owy ch ogniw wy miaru spra - wiedliwoœci. Gdy policja decyduje siê na postawienie zarzut ów spraw cy, spra wa tra fia do pro kur atu ry. Pro kurator do kon uje anal izy akt spra wy pod k¹tem mo l iwoœci po pier ania aktu os kar enia przed s¹dem lub te ewent ual - nego jej umor zenia. W pro ces ie de cyz yjnym pro kur ator kieruje siê zasadami wymienionymi w Kodeksie Prokura - torów Ko rony oraz sto suje przy ocen ie spra wy dwa opi - sa ne tam te sty: test do wod owy oraz test in ter esu pu - blicz nego. W ra zie ko niecznoœci zmia ny zarz utów, któ - re mo na zmie niaæ tak e w toku pro cesu a do cza su pod - jêcia przez s¹d de cyz ji ko ñcz¹cej po stê powanie lub w ob lic zu mo li woœci umor zenia spra wy (post êpowanie mo na umor zyæ do chwi li przed staw ienia spra wy s¹ do - wi) pro kur atu ra kon takt uje siê z po licj¹, kt óra prze prow a - dza, jeœli sama uzna to za za sadne, do datk owe cz yn no œci do wod owe. Test do wod owy (ang. evi dent ial test) po zwa - la na ustalenie, czy materia³ dowodowy zgromadzony w sprawie daje realistyczn¹ perspektywê skazania spraw cy za pope³nio ny czyn oraz ja kie ok olicznoœci prze - maw iaj¹ na jego korz yœæ. Per spekt ywa, o któ rej mowa, opiera siê na prze wid ywa niu post êpo wan ia ³awy przy - siêg³ych w ob lic zu przed staw iony ch im w cza sie roz - prawy dow odów. Oznac za to, e wystê puje wiêks ze prawdopodobieñstwo, e ³awa przys iêg³ych (ang. jury) lub sêdzi owie po koju (ang. ben tch of ma gis trates), kie ruj¹c siê zasadami prawa, wydadz¹ wyrok skazuj¹cy oskar o - nego, ni e taki wy rok nie za padn ie. Prze prow adz enie te stu do wod owe go po zwala pro kur ato rowi odpowie - dzieæ na dwa py tan ia. Po pierw sze, czy ze brane do wody mo na bêdzie wykorzystaæ w s¹dzie? Szczególnie istotny jest tu pro blem dop uszczalnoœci dow odów. Pew na ich gru pa z uwagi na sztyw ne regu³y do wod owe obo wi¹ - zuj¹ce w prawie Zjednoczonego Królestwa nie mo e stan owiæ dowo dów i s¹d od maw ia ich do puszc zenia. Na - le y do niej na przyk³ad dowód poœredni z przes³uchan ia œwiad ka, który nie by³ na miej scu zda rzen ia, a o spra wie us³ysza³ od in nej osoby. Po dru gie, czy do wody te s¹ wia - ryg odne? W kwe stii wia rygodnoœci dow odów ko deks wy lic za sze reg oko licznoœci, któ re win ny byæ bra ne pod uwagê przez prokuratora prowadz¹cego sprawê. Dotyczy to mi êdzy in nymi wi ary godnoœci osob owy ch Ÿróde³ do - wod owy ch. Wy maga to do brej wspó³pra cy z bieg³ymi s¹do wymi. Je eli ma ter ia³ spra wy nie spe³nia kryt eriów te stu do wod owe go, po stê pow anie zo staje umor zone [8]. Za sadn icz ym elem entem, który bieg³y mo e wnie œæ do wyjaœnienia danej sprawy, jest interpretacja uzys - kanych obserwacji. Niew¹tpliwie najwa niejsz¹ cech¹ nauk s¹do wych jest bo wiem zdobycie do datk owy ch in - form acji z prze prow adz onych w la bor ato rium ba dañ. Na ogó³ opin ia bieg³ego jest uwa ana jako bezp oœrednie opis anie wnio sków wy nik aj¹cych z prze prow adz onych badañ, dla tego ostat eczne wnio ski bieg³ego co do inter - pretacji wa rtoœ ci do wodu ro zwa ane s¹ na ko ñco wym etap ie do chod zenia w spra wie b¹dÿ na sali s¹do wej. Ten pogl¹d w ostatn ich la tach zo sta³ dok³ad nie za nal izo wany przez Fo rens ic Scien ce Se rvice (FSS) w ra mach pro jektu Wstêpne oszacowanie sprawy i jej interpretacja CAI (ang. Case As sessment and In terp reta tion) [6]. G³ów nym ce lem pro jektu CAI by³o zop tym ali zow anie kos ztów, ja - kie ofer owa³ FSS dla swo ich odb iorców. Zak³ada on part - nerstwo pomiêdzy zleceniodawc¹ a wykonawc¹ opinii w celu jak naj leps zego wy kor zyst ania œr odk ów fi nans o - wy ch prze znac zony ch na wy kon anie eks pert yzy. Kon - sek wencj¹ ta kiego po dejœ cia jest po dkre œlenie przy datno œci za bezp iecz onych do wodów do wy jaœn ienia oko - licz noœci spra wy we wszyst kich mo liw ych aspekt ach. Po zwala to na rozs¹dny dia log pom iêdzy od biorc¹ opin ii a jej wy kon awc¹ co do ko szt ów i mo li wych do uzys - kania wnio sków. Co wiêc ej, u³atwia to po staw ienie hi - pot ez do tycz¹cych mo liwoœci uzys kania do wodu wyni - kaj¹cych z perspektywy wyjaœnieñ podawanych przez po dejr zane go oraz z punk tu wi dzen ia os kar enia. CAI re - kom endu je punkt wi dzen ia, w wy niku kt óre go in terp reta - cja i oszacowanie wartoœci z badania dowodów rzeczowych po winno byæ g³ów nym punk tem za int ere sow ania wy kon uj¹cego op iniê. Je eli za stos uje siê to po dejœ cie, posiadaj¹c znajomoœæ zarysu okolicznoœci zdarzenia, to mo na wysnuæ pewne wnioski, jeszcze zanim ostateczne badania zostan¹ zakoñczone. Zasadnicz¹ rolê pe³ni tu zastosowanie elementarnych zasad statystyki. Z jednej stro ny jest ona bardzo u yteczna na etapie dochodzenia, zanim jeszcze zostanie przeprowadzona ekspertyza,