COMPLEX DNA PROFILES

Interpretation of convoluted DNA profiles utilising empirical forms and a procedure to assess their robustness

Interpretation of complex DNA profiles using empirical models and a method to measure their robustness

Introduction

Using traditional methods, it is only possible to report a DNA profile that actually matches the suspect in whole or in part. Consequently, the probative value given always has a likelihood ratio (LR) greater than one. However, DNA profiles are often ambiguous—they may be partial, with alleles missing under prosecution (Hp) propositions; they may be mixtures; stutters may interfere with the interpretation. Every DNA scientist will routinely make decisions on whether to report a profile in the context of missing alleles, or additional alleles in the profile that do not match the suspect. Expert opinion is used to carry out the assessment, but this can lead to reporting inconsistencies where some scientists may apply a probative value to a result, whereas others may be more 'cautious' and provide an inconclusive result that neither includes nor excludes the suspect.

Consequently, there is no longer a need for an 'inconclusive' category for reporting purposes that is based on perceived complexity of the result. In principle any profile (with any number of potential contributors) can be probabilistically evaluated against any set of hypotheses.

We show that there is no need for an 'inconclusive' category for reporting purposes that is based on perceived complexity of the result because in principle any profile can be probabilistically evaluated against any set of hypotheses.

How robust is the answer

Once a likelihood ratio has been calculated, then case-specific Tippett tests provide insight into the robustness of the test itself. Specifically, we are most interested in answering the question: “if the suspect is not a contributor to the crime-stain, how likely is it that a LR of similar magnitude would be reported if a random man was the contributor” We use computer simulation to address this issue. Instead of calculating the LR relative to the suspect and conditioned profiles under the prosecution hypothesis (Hp), we replaced these profiles with random men - which are the proposition under the defence hypothesis (Hd). We demonstrated that the resulting LRs were substantially less than one and the chance that random man would give probative evidence was negligible in the examples we discuss.

We show that these principles can be usefully applied to complex cases that hitherto could not have been reported beforehand.

The role of expert judgment in relation to DNA profiling evidence

Empirical guidelines are routinely used in classical DNA mixture interpretation to designate alleles and to make decisions on the numbers of contributors. Expert decisions tend to be binary, i.e. a probability of an event is either zero or one. For example:

Heterozygote pairing: Heterozygote balance measures the relative sizes, in terms of peak height (Ht) or peak area, of two alleles. Hence heterozygote balance can be defined as

If the smaller of two alleles is within 60% (Hb > 60%) of the major allele then this means that they are complementary and it is possible that they ...