CONSIDERATION OF EVIDENCE
In many situations, particularly in forensic science, there is a need to consider one piece of evidence against one or more other pieces of evidence. For instance, it may be desirable to compare a sample collected from a crime scene with a sample collected from a person, with a view to linking the two by comparing the characteristics of their DNA, particularly by expressing the strength or likelihood of the comparison made, a so called likelihood ratio. The method provides a more accurate or robust method for establishing likelihood ratios through the definitions of the likelihood ratios used and the manner in which the probability distribution functions for use in establishing likelihood ratios are obtained The methods provide due consideration of stutter and/or dropout of alleles in DNA analysis, as well as taking into consideration one or more peak imbalance effects, such as degradation, amplification efficiency, sampling effects and the like.
1 . A method of comparing a test sample result set with another sample result set, the method comprising:
providing information for the first result set on the one or more identities detected for a variable characteristic of DNA;
providing information for the second result set on the one or more identities detected for a variable characteristic of DNA; and
comparing at least a part of the first result set with at least a part of the second result set.
2 . A method according to claim 1 , in which the method comprises:
a consideration of the size of the alleles in one or more loci; and/or
a consideration of the size of the alleles across two or more loci; and/or
a consideration of the identity of the loci; and
the provision of an adjustment arising there from.
3 . A method according to claim 2 , wherein the adjustment is provided to account for degradation and/or amplification efficiency and/or inhibition.
4 . A method according to claim 1 , wherein the method includes the use of a likelihood ratio and the numerator and/or denominator are presented in a form based around the core pdf:
ƒ( c (l) |g 1 (l) ,ω,χ (l) )
where g 1 (l) is the genotype of the donor of sample c (l) , χ (l) denotes the quantitative measure for the locus i and ω is the mixing proportion; and/or
ƒ( c (l) |g 1 (l) ,g 2 (l) ,ω,χ (l) )
where g 1 (l) is one of the genotypes of the donor of sample c (l) , g 2 (l) is another of the genotypes of the donor of the sample c (l) , and χ (l) denotes the quantitative measure for the locus i and ω is the mixing proportion.
5 . A method according to claim 1 , wherein the method of comparing includes a comparison in the form of a likelihood ratio in which the denominator is of the form:
Den =ƒ( c|g s ,H d )
where
c is the first or test result set from a test sample;
g s is the second or another result set;
H d is one hypothesis; and
the denominator includes the factor ƒ(c (l) |g 1 (l) ,ω,χ (l) ), where g 1 (l) is the genotype of the donor of sample c (l) , χ (l) denotes the quantitative measure for the locus i and ω is the mixing proportion; and/or the factor ƒ(c (l) |g 1 (l) ,g 2 (l) ,ω,χ (l) ), where g 1 (l) is one of the genotypes of the donor of sample c (l) , g 2 (l) is another of the genotypes of the donor of the sample c (l) , and χ (l) denotes the quantitative measure for the locus i and ω is the mixing proportion.
6 . A method according to claim 1 , wherein the method provides the use of a Gamma distribution as the form of the distribution used to represent a peak in the method, and the Gamma distribution is defined by one or more parameters, including a shape parameter α and/or a rate parameter β.
7 . A method according to claim 4 , wherein the method provides that the construction of ƒ(c (l) |g 1 (l) ,g 2 (l) ,ω,χ (l) ) includes two or more steps:
in a first step, the a parameters for the alleles and stutters of genotypes g 1 (l) and g 2 (l) being calculated;
and/or
in a second step, the factors of the probability density functions, pdf's, being determined.
8 . A method according to claim 1 , wherein the method provides that the genotypes of the major and minor donors in the mixture are denoted as g l(i) ={a gi,1 , a gi,2 }, i=1,2 respectively, and the base pair counts of the alleles are denoted with the same indices, bp gi,j is the base count of a gi,j , and in which a total of eight α parameters are obtained of the form:
A g1,g2 (l) {α a,g1,1 (l) ,α s,g1,1 (l) α a,g1,2 (l) ,α s,g1,2 (l) α a,g2,1 (l) α a,g2,1 (l) α s,g2,1 (l) α a,g2,2 (l) α s,g2,2 (l) },
where a i,gi,k (l) is the α parameter for either an allele (i=a) or a stutter (i=s), for the major (j=1) or the minor (j=2) donors for the first (k=1) or second (k=2) allele of the corresponding genotype and the method defined for the calculation of α a,g1,1 and α s,g1,1 is used in an equivalent manner to provide the parameters for other alleles and/or loci and/or genotypes.
9 . A method according to claim 1 , wherein the method provides that the major donor contributes with (ω×100) % of the DNA and in which the method provides for the calculation of ωχ/bp g1,1 and where:
if the number from this calculation is greater than the upper limit of the dropout region for alleles, then the α parameter is calculated using equation:
α
i
(
l
)
=
κ
1
,
i
(
l
)
κ
2
(
l
)
×
χ
(
l
)
bp
a
(
l
)
,
i
=
a
,
s
and
β
a
(
l
)
=
β
s
(
l
)
=
1
κ
2
(
l
)
;
and/or
if the number from this calculation is otherwise, then the α parameter is preferably calculated using equation
α
i
(
l
)
=
intercept
+
slope
x
χ
(
l
)
bp
a
(
l
)
10 . A method of comparing a first test, sample result set with a second another sample result set, the method comprising:
providing information for the first result set on the one or more identities detected for a variable characteristic of DNA;
providing information for the second result set on the one or more identities detected for a variable characteristic of DNA; and
wherein the method uses in the definition of the likelihood ratio the factor:
ƒ( c (l) |g 1 (l) ,ω,χ (l) ),
where g 1 (l) is the genotype of the donor of sample c (l) , χ (l) denotes the quantitative measure for the locus i and ω is the mixing proportion and/or
ƒ( c (l) |g 1 (l) ,g 2 (l) ,ω,χ (l) ,
where g 1 (l) is one of the genotypes of the donor of sample c (l) , g 2 (l) is another of the genotypes of the donor of the sample c (l) , and χ (l) denotes the quantitative measure.