Purpose Of Statistics And Population Considerations

In Chapter 15 on short tandem repeat (STR) interpretation, we concluded with a section entitled 'to match or not to match - that is the question.' If a DNA profile from a suspect does not match the evidence from a crime scene (and the testing has been performed properly), then we can reliably conclude that the individual in question did not contribute the biological sample recovered from the crime scene.

However, the more interesting outcome of a DNA profile comparison is what to conclude when the profiles between suspect and evidence match. Are they from the same individual or is there someone else out there who might just happen to match the evidence in question? Since we do not have the luxury of access to DNA profiles of everyone living on planet Earth, we must use smaller population data sets to extrapolate the possibility of a random match. As will be described in more detail in Chapter 20, allele frequencies are collected from various ethnic/racial sample sets, such as contained in Appendix II. Based on their allele frequencies from validated databases, population genetic principles are applied to infer how reasonable it is that a random, unrelated individual could have contributed the DNA profile in question (see Chapter 21).

It is important to distinguish between unrelated and related individuals in assumptions being made for the calculations that follow. Obviously related individuals have DNA profiles that are more similar than unrelated individuals who are compared. In most equations that will be used in Chapters 19-22, we will be assuming that unrelated individuals are involved. In Chapter 23, we consider paternity testing and other kinship scenarios where closely related individuals are being studied.

Of the three possible outcomes of a DNA test - 'no match', 'inconclusive', or 'match' between samples examined - only the third requires statistics. Statistics attempt to provide meaning to the match. These match statistics are usually provided in the form of an estimate of the random match probability or in other words, the frequency for the particular genotype (DNA profile) in a population. However, different laboratories may use different methods for calculating the statistical topics discussed in this book.

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