Confidence interval for a 2-group median difference

Computes a distribution-free confidence interval for a difference of population medians in a 2-group design. Tied scores within each group are assumed to be rare.

Usage

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(32.1, 39.8, 26.3, 35.0, 43.1, 27.0, 40.9, 37.4, 34.0, 29.2)
y2 <- c(36.8, 44.0, 47.1, 42.7, 49.0, 39.6, 46.2, 31.6, 33.1, 48.4)
ci.median2(.05, y1, y2)
#>  Median1 Median2 Median1-Median2       SE        LL         UL
#>     34.5   43.35           -8.85 4.494993 -17.66002 -0.0399751

# Should return:
#  Median1 Median2 Median1-Median2       SE        LL         UL
#     34.5   43.35           -8.85 4.494993 -17.66002 -0.0399751