Confidence interval for a 2-group median difference

Source: R/statpsych1.R

Computes a distribution-free confidence interval for a difference of population medians in a 2-group design.

ci.median2(alpha, y1, y2)

Arguments

alpha

alpha level for 1-alpha confidence

y1

vector of scores for group 1

y2

vector of scores for group 2

Value

Returns a 1-row matrix. The columns are:

  • Median1 - estimated median for group 1

  • Median2 - estimated median for group 2

  • Median1-Median2 - estimated difference of medians

  • SE - standard error

  • LL - lower limit of the confidence interval

  • UL - upper limit of the confidence interval

References

Bonett DG, Price RM (2002). “Statistical inference for a linear function of medians: Confidence intervals, hypothesis testing, and sample size requirements.” Psychological Methods, 7(3), 370--383. ISSN 1939-1463, doi:10.1037/1082-989X.7.3.370 .

Examples

y1 <- c(32, 39, 26, 35, 43, 27, 40, 37, 34, 29)
y2 <- c(36, 44, 47, 42, 49, 39, 46, 31, 33, 48)
ci.median2(.05, y1, y2)
#>  Median1 Median2 Median1-Median2       SE        LL          UL
#>     34.5      43            -8.5 4.316291 -16.95977 -0.04022524

# Should return:
# Median1 Median2 Median1-Median2       SE        LL          UL
#    34.5      43            -8.5 4.316291 -16.95977 -0.04022524