size.ci.median2: Sample size for a 2-group median difference confidence...

In statpsych: Statistical Methods for Psychologists

Sample size for a 2-group median difference confidence interval

Description

Computes the sample size for each group required to estimate a population median difference with desired confidence interval precision in a 2-group design. Set the variance planning value to the largest value within a plausible range for a conservatively large sample size. The sample size requirement depends on the shape of the distribution. Select one of the four distribution options (Normal, Logistic, Laplace, Exponential) that approximates the most likely distribution shape in the planned study. Select the Normal distribution for a conservatively large sample size requirement. Set R = 1 for equal sample sizes.

Usage

size.ci.median2(alpha, var, w, R, dist)

Arguments

alpha	alpha level for 1-alpha confidence
var	planning value of average within-group variance
w	desired confidence interval width
R	n2/n1 ratio
dist	set to 1 for Normal distribution (skew = 0, kurtosis = 3) set to 2 for Logistic distribution (skew = 0, kurtosis = 4.2) set to 3 for Laplace distribution (skew = 0, kurtosis = 6) set to 4 for Gamma(5) (skew = .89, kurtosis = 4.2) set to 5 for Exponential distribution (skew = 2, kurtosis = 9)

Value

Returns the required sample size for each group

References

\insertRef

Bonett2002statpsych

Examples

size.ci.median2(.05, 37.1, 5, 1, 1)

# Should return:
# n1  n2
# 72  72

size.ci.median2(.05, 37.1, 5, 2, 4)

# Should return:
# n1  n2
# 51 102

statpsych documentation built on June 10, 2025, 5:11 p.m.