Statistics Flashcards
What are the 5 requirements to be in HWE?
- Random mating
- Large population size
- No mutation
- No natural selection
- No gene flow
What is the HW equation?
p^2 + 2pq + q^2 = 1
What is theta?
A correction factor used to address the issue of increased homozygosity from a population substructure; a.k.a. co-ancestry coefficient, the relatedness factor, the inbreeding coefficient, or fixation index
What would happen if you set the theta too high or too low?
Setting θ too high will overcorrect and possibly reduce the statistic inappropriately.
Setting θ too low will under-correct and possibly reduce the statistic insufficiently.
What is the product rule?
It is the statistical principle allowing unlinked or independent events to be combined using multiplication.
Define probability.
It is a mathematical relationship between the number of times an event is observed compared to the total number of events possible; the value is always between 0 and 1.
What is a random match probability?
It is the estimated frequency at which a particular STR profile would be expected to occur in a population as determined by the allele frequencies from that population group.
Do we report the actual RMP?
No, the Random Match Probability itself is not reported but the frequency of a random match is reported. The frequency is 1/RMP.
How can we use the HW equation if we’re not in HWE?
Exact tests were performed to determine if the observed allele frequencies deviated significantly from HWE. They determined:
-No significant deviation from HWE
-Loci are sufficiently discriminatory
-There are significant differences in allele frequencies among different population groups
-The loci are sufficiently independent (no evidence of association), so the product rule is valid
When is a minimum allele frequency applied?
It is used both for unobserved alleles and raising frequencies that fall below the MAF
What is a minimum allele frequency?
It is the minimum allowable frequency within a population group and is based upon the size of the population sampled
What is the most common equation to calculate the minimum allele frequency and what does each part of the equation refer to?
5/2n;
The numerator is the minimum number of times that an allele should be seen for a reliable frequency and is 5 for this formula.
The denominator is 2 multiplied by the size of the database to account for all observed alleles within a locus.
What is the purpose of statistical analysis in forensic DNA?
Statistical calculations are performed following proper interpretation on evidentiary DNA profiles to provide an assessment of the significance of (or give weight to) an inclusion
What is a likelihood ratio?
It is the ratio of two probabilities of the same event under different and mutually exclusive hypotheses; it is a statistic that is specific to the observed evidence in the case and comparing specific individuals to that evidence
Does LSPCL perform restricted or unrestricted likelihood ratios utilizing the Popstats software?
Unrestricted
Why are we required to perform statistical analysis?
It is an FBI QAS requirement (Standard 9.10) & ISO/AR requirement; additionally there is guidance from the NRC II and SWGDAM regarding statistical analysis in support of inclusions
What is a population database?
It is a collection of observed alleles and associated frequencies for tested populations
Why is the “in how many world’s would you need to have to see this profile again” fallacy wrong?
The RMP statistic is not relevant to the population size of the world. It is an estimate of the probability of observing the profile in an unrelated random individual. It refers to the outcome over one trial, not how many times that outcome is expected over several trials. Meaning, every time you select an unrelated random individual, there is a 1 in ### chance this profile would be observed again. To simplify this to the die example, every time you roll a die, there is a 1 in 6 chance of rolling a 1.
Why is random mating significant to HWE?
It prevents inbreeding or the occurrence of a population substructure
Why is a large population size significant to HWE?
It ensures allele frequency is not changed through genetic drift
Why is no mutation significant to HWE?
It avoids introducing new alleles into a population
Why is no natural selection significant to HWE?
There should be no selection of stronger genes over inferior genes which would cause allele frequencies to change if alleles were being favored or lost
Why is no gene flow significant to HWE?
Having gene flow would increase variability in the gene pool
What is Hardy-Weinberg Equilibrium (2 parts)?
- A mathematical relationship between allele frequencies and genotype frequencies
- The principle of a perfectly balanced population where the genetic variation remains constant between generations in the absence of disturbing factors
Does theta generally increase or decrease the statistic?
Decrease
