Defense Proposed Likelihood Ratio in DNA Evidence Jury Instruction

During this trial you heard testimony about DNA evidence.  The DNA analyst reported to you a number using a statistic called a likelihood ratio.  This is a very different statistic than what we ordinarily use in everyday life and should not be confused with statistics like the probability that it will rain next Monday or that a random person might match a DNA profile. This is a different statistic with a very different meaning.

There are certain misunderstandings that can come from a likelihood ratio.  To ensure there is no confusion the following points must be emphasized:

The likelihood ratio does not say how probable it is that Mr. ______ left DNA on an item.

The likelihood ratio likelihood ratio does not say how likely it is that Mr. _____ is guilty.

Finally, the likelihood ratio cannot tell you how DNA got onto an item whether it is through handling it, indirect transfer or laboratory contamination.

What is the likelihood ratio then?  The likelihood ratio is the relative probability of observing the DNA evidence under two competing hypotheses. Two possible explanations for the evidence were compared, but they are not the only possible explanations for the evidence, and the Likelihood Ratio cannot tell you how objectively probable either scenario is. It is important to remember that both hypotheses could be wrong, but one may still be a better explanation than the other. When compared, any two hypotheses will produce a Likelihood Ratio

In this case the two hypotheses compared are that the contributors to the DNA are the DEF and two random unrelated individuals or instead, three random unrelated individuals. No other possible hypotheses were considered. Neither of those hypotheses posed the question as to whether Mr. _____ handle the _____ or whether Mr. _____ is guilty.  That statistic cannot tell you whether Mr. ____ contributed to the DNA mixture on the _____.

Like with any scientific test, there are always risks of false positives and false negatives. If you do not believe that any hypothesis proposed by the government when calculating this statistic is true, you may not speculate as to what the Likelihood Ratio would be under any alternative hypothesis that was not tested.

The likelihood ratio is not commonly used in day-to-day life and it is understandable if there is confusion around its meaning. This evidence should be weighed with caution rather than assuming the validity of a statistic and should be weighed along with all the other evidence, or lack of evidence, in determining whether the government proved their case beyond a reasonable doubt.