SE2 <- function(x, y)
{
# SE von x und y zusammen
# fuer einen 2-sample t-test
nx = length(x)
ny = length(y)
sx = sd(x)
sy = sd(y)
num = ((nx - 1) * sx^2) + ((ny - 1) * sy^2)
den = nx + ny - 2
sqrt(num/den) * sqrt(1/nx + 1/ny)
}

x = c(20, 15, 19, 22, 17, 16, 23, 18, 20)
y = c(18, 15, 17, 24, 15, 12, 14, 11, 13, 17, 18)