Independent Events
Definition
Two events where the occurrence of one provides no information about the other: P(A and B) = P(A) x P(B). In forensic STR profiling, loci on different chromosomes are treated as independent, permitting allele frequencies to be multiplied across loci to compute a profile probability.
- Field
- Probability theory, forensic statistics
- Rule
- P(A and B) = P(A) x P(B)
- Forensic application
- Multiplying STR allele frequencies across loci
- Basis for STR use
- Loci on different chromosomes assort independently
Common questions
Why does STR profiling rely on treating loci as independent rather than measuring joint frequencies directly?+
Measuring the joint frequency of a full multi-locus profile directly in a reference population would require an impossibly large sample, so the product rule lets analysts combine single-locus frequencies, each estimated from a manageable database, into a profile-wide probability.
What happens to a match probability calculation if two loci are not actually independent?+
If loci are linked or the population has substructure, multiplying their frequencies can overstate rarity, which is why forensic labs validate linkage equilibrium in their reference databases and apply population-genetics corrections such as theta when independence is in doubt.
Related terms
- Complement
- A group of heat-labile serum proteins (approximately 30 proteins, designated C1 through C9 in the classical pathway) that can be activated by...
- Conditional Probability
- The probability of event A given that event B has already occurred, written P(A|B). Conditional probability is the basis of the multiplication...
- Event
- Any subset of the sample space to which a probability is assigned. An event may be a single outcome (this particular allele)...
- Mutually Exclusive Events
- Two events that cannot both occur in the same trial. If a fibre is classified as cotton, it cannot simultaneously be classified...
- Sample Space
- The set of all possible outcomes of an experiment. In forensic genetics, the sample space for a single allele call is the...