Sample Space
Definition
The set of all possible outcomes of an experiment. In forensic genetics, the sample space for a single allele call is the set of all alleles observed in the relevant population database. The probability of the entire sample space equals 1 by the second Kolmogorov axiom.
- Definition scope
- All possible outcomes of an experiment
- Forensic genetics example
- All alleles in the population database
- Governing rule
- Second Kolmogorov axiom, total probability equals 1
Common questions
Why does defining the sample space matter before calculating a match probability?+
Every probability calculation is only meaningful relative to a stated sample space, so if the wrong or an incomplete set of possible alleles is used, the resulting match statistic will misrepresent the true rarity of the observed profile.
How does the choice of population database affect the sample space in a real case?+
Different reference populations carry different allele frequencies, so the same genotype can yield a different match statistic depending on which population's allele frequency table is used to define the relevant sample space.
What happens if an observed allele falls outside the assumed sample space?+
An allele not represented in the reference database requires the analyst to apply a conservative correction or minimum frequency estimate, since assigning it zero probability would be both statistically and legally indefensible.
Related terms
- Complement
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- 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)...
- Independent Events
- Two events where the occurrence of one provides no information about the other: P(A and B) = P(A) x P(B). In forensic...
- 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...