expectedLR: Expected likelihood ratio

In forrel: Forensic Pedigree Analysis and Relatedness Inference

Expected likelihood ratio

Description

This function computes the expected LR for a single marker, in a kinship test comparing two hypothesised relationships between a set of individuals. The true relationship may differ from both hypotheses. Some individuals may already be genotyped, while others are available for typing. The implementation uses oneMarkerDistribution() to find the joint genotype distribution for the available individuals, conditional on the known data, in each pedigree.

Usage

expectedLR(numeratorPed, denominatorPed, truePed = numeratorPed, ids, marker)

Arguments

numeratorPed	A ped object or a list of such.
denominatorPed	A ped object or a list of such.
truePed	A ped object or a list of such.
ids	A vector of ID labels corresponding to untyped pedigree members. (These must be members of all three input pedigrees).
marker	The name or index of a marker attached to numeratorPed. Alternatively, a standalone marker object compatible with numeratorPed.

Value

A positive number; the expected LR.

Examples

#---------
# Curious example showing that ELR may decrease
# by typing additional reference individuals
#---------

# Numerator ped
num = nuclearPed(father = "fa", mother = "mo", child = "ch")

# Denominator ped: fa, mo, ch are unrelated
den = singletons(c("fa", "mo", "ch"), sex = c(1, 2, 1))

# Scenario 1: Only mother is typed; genotype 1/2
p = 0.9
m = marker(num, mo = "1/2", afreq = c("1" = p, "2" = 1-p))
expectedLR(num, den, ids = "ch", marker = m)

1/(8*p*(1-p)) + 1/2  # exact

# Scenario 2: Include father, with genotype 1/1
genotype(m, id = "fa") = "1/1"
expectedLR(num, den, ids = "ch", marker = m)

1/(8*p*(1-p)) + 1/(4*p^2)  # exact

forrel documentation built on Oct. 6, 2026, 5:06 p.m.