Effective Size per Foils — eff_size_per

A package-specific count of members whose choice-proportion intervals include chance guessing. This is distinct from Malpass' continuous effective size.

Usage

eff_size_per_foils(lineup_vec, target_pos, k, conf = 0.95, R = 1000)

Arguments

lineup_vec: A numeric vector of lineup choices
target_pos: A numeric vector indexing all lineup members
k: Number of members in lineup. Must be specified by user (scalar).
conf: Confidence level for the intervals, e.g. 0.95 for 95 percent intervals. Defaults to 0.95. May be specified by user (scalar).
R: Number of bootstrap replications. Defaults to 1000.

Value

A single integer: the number of lineup members whose bootstrapped confidence interval for the proportion choosing them includes the chance value \(1/k\). These are the members that a mock witness chose at a rate not distinguishable from guessing.

Details

Because the result is a count of members rather than a continuous quantity, it changes only in whole steps as conf varies, and it is flat across ranges of conf in which no member's interval crosses \(1/k\). A value that does not move when conf is changed from, say, 0.95 to 0.99 is therefore expected behaviour and not a sign that the argument is being ignored. Widening the interval can only ever increase the count.

Intervals are computed by lineup_boot_allprop, and are not corrected for the fact that k of them are examined at once.

References

Malpass, R S. (1981). Effective size and defendant bias in eyewitness identification lineups. Law and Human Behavior, 5, 299-309.

Tredoux, C. G. (1998). Statistical inference on measures of lineup fairness. Law and Human Behavior, 22(2), 217-237.

Examples

#Data:
lineup_vec <- round(runif(100, 1, 6))
target_pos <- c(1, 2, 3, 4, 5, 6)

#Call:
eff_size_per_foils(lineup_vec, target_pos, 6)
#> [1] 5
eff_size_per_foils(lineup_vec, target_pos, conf = 0.95, 6)
#> [1] 5