Profile ceiling principle
In some countries, such as the UK, the approach has been to use a match probability of 1 in a billion (1000000000). This approach is highly conservative . It does have the advantage that individual profile frequencies do not have to be calculated because the value used is much lower than the most common profile frequency, even if conservative corrections are incorporated .
WHICH POPULATION FREQUENCY DATABASE SHOULD BE USED?
Table 8.3 The effect of different correction methods on the profile frequency calculated in Table 8.1. With this profile, applying a minimum allele frequency of 0.0125 would have no impact because the rarest allele frequency is 0.025
Which population frequency database should be used?
In some cases, the ethnic origins of material recovered from the crime scene are known: for example, if a woman has been sexually assaulted she can normally describe the as- sailant as white, black, Asian, etc. In such a case, for example, if the assailant was described as white, then it would be logical to use a white Caucasian allele frequency database to calculate the profile frequency. In other contexts, there may be no informa- tion about who could have left the material at the crime scene. In countries or regions having substantial populations with different ethnic backgrounds, a common practice is for the profile frequency to be calculated using an allele database for each major population group, and to use the most conservative profile frequency. If we take the ex- ample from Table 8.1, the allele frequency data used is from a white Caucasian database (USA); if we recalculate with allele frequency data representing an African American population we get a profile frequency of 3.36 × 10−16, which is over 200-times less frequent than when we use the Caucasian frequency data. In this case it is clear that the Caucasian data provides a frequency estimate that is more conservative.
The methods that are employed for the correction of profile frequencies vary widely between different judicial systems and even different laboratories within the same judi- cial system. The allele ceiling principle, Balding correction, 95% confidence interval to correct for sampling error, accounting for population sub-division using theta, and the profile ceiling principle have all been used in forensic casework to calculate profile frequencies. For the profile that we have been using as an example in this chapter, the effect of the different correction methods can be seen in Table 8.3. It should be noted that the impact of the different correction methods will vary depending on the individual profile and the size of the allele frequency database. The end result of analysing a profile is to produce a profile frequency, which is an estimate. Incorporating one or more of the correction factors into the profile frequency estimates reduces the chances of overstating the DNA evidence.
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