APPROACHES TO SOLVING THE MATCH PROBABILITY PROBLEM

• Jurisdiction: Australia

APPROACHES TO SOLVING THE MATCH PROBABILITY PROBLEM

Overview ....................................................................................................... [80A.600]

What assumptions are made? ...................................................................... [80A.620]

Which population? ........................................................................................ [80A.640]

Drawing conclusions from a population database........................................ [80A.660]

[80A.600] Overview

Blood group and polymorphic enzyme and protein typing evidence (from systems such as ABO, Rhesus, Haptoglobin, Human Leucocyte Antigen) was traditionally presented along with a frequency of occurrence obtained from databases. No one claimed that blood groups were unique. When evidence based on, say, two different blood group systems was presented in forensic reports, an estimate was made of the rarity of that combination. Such estimates were made by calculation from databases of blood group typings from which relative frequencies of genotypes could be found. For example, Haptoglobin (Hp) type 2-1 occurs in about 10% of the population (f2_1 = 0.1); ABO type AB occurs in about 3% (fAB = 0.03). The expected relative frequency of the combination of Hp type 2-1 and ABO type AB could be calculated by multiplying the individual observed relative frequencies:

f2-1×fAB = 0.01×0.03

= 0.003

= 1 in 333

This multiplication of two relative frequencies to find their product is known as the Product Rule: National Research Council (1992).

The Product Rule is still considered appropriate in civil and coronial work (paternity testing and disaster victim identification; recommendation of Australasian Scientific Working Group - Forensic DNA Statistics 2010) and in some other jurisdictions.

Match probability calculations using the Product Rule make several assumptions which have been questioned by academics: Lewontin and Hartl (1991); Sudbury, Marinopoulos and Gunn (1993) and in...