Essen-Moller W
Definition
The formula for probability of paternity derived by Erik Essen-Moller in 1938 and widely adopted by forensic laboratories. W = CPI / (CPI + 1) when the prior probability is 0.5. Still the standard conversion used for serological and DNA paternity reports.
- Originator
- Erik Essen-Moller
- Year
- 1938
- Formula
- W = CPI / (CPI + 1), assuming prior probability 0.5
- Field
- Paternity probability from serological or DNA evidence
Common questions
What does the W value from the Essen-Moller formula represent?+
It converts the Combined Paternity Index into a posterior probability of paternity, expressed as a percentage, assuming the alleged father is a priori equally likely to be the true father or not.
Why does the formula assume a prior probability of 0.5?+
A 0.5 prior treats paternity as equally likely before testing, which is a neutral default. If case circumstances suggest a different prior, such as other evidence pointing toward or away from the alleged father, W should be recalculated with that prior rather than the default.
Is the Essen-Moller W the same as the Combined Paternity Index?+
No. The CPI is a likelihood ratio built from the genetic data alone, while W is the CPI converted into a probability statement after applying a prior, so W depends on an assumption the CPI itself does not require.
Related terms
- Combined Paternity Index (CPI)
- The product of all individual paternity indices across all marker systems tested. Assumes independence of the marker systems. The CPI is a...
- Exclusion Probability (W)
- The proportion of the male reference population that would be excluded as the biological father by the marker panel tested. A high...
- Obligatory Paternal Allele
- The allele a child must have received from the biological father, identified by subtracting the mother's alleles from the child's genotype. The...
- Paternity Index (PI)
- A likelihood ratio that compares the probability of the observed serological profile if the alleged father is the biological father to the...
- Probability of Paternity (W)
- The posterior probability that the alleged father is the biological father, calculated from the CPI and a prior probability using Bayes' theorem....