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Population Genetics and Statistics for Forensic Analysts

Likelihood Ratio Continued

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Calculations may also be performed to determine the probability of seeing a certain profile given that it has been seen once already, which is a factor to be considered in regard to the integrity of database information.

This type of conditional probability uses the equations 4.10a and 4.10b from NRC II (1996 National Research Council Report) report.

Read about equation 4.10a in NRC.

Read about equation 4.10b in NRC.

  • For homozygotes use:
    P(Ai Ai |Ai Ai) = [2θ+(1-θ)pi][3θ+(1-θ)pi]/[(1+θ)(1+2θ)]
  • For heterozygotes use:
    P(Ai Aj |Ai Aj) = 2[θ+(1-θ)pi][θ+(1-θ)pj]/[(1+θ)(1+2θ)]


where θ is the average of the parameters of θ over all genotypes

As with forensic samples, a five-event minimum allele frequency should be used for rare alleles. 

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