nrep = 1000

## The second instrument has twice the variance, but we get to 
## use twice the sample size.
nx = 10
vx = 1
ny = 20
vy = 2

## Generate the data for the first instrument.
D = rnorm(nrep*nx, sd=sqrt(vx))
X = array(D, c(nrep,nx))
MX = apply(X, 1, mean)

## Generate the data for the first instrument.
D = rnorm(nrep*ny, sd=sqrt(vy))
Y = array(D, c(nrep,ny))
MY = apply(Y, 1, mean)


## Concatenate the means into a single vector.
M = c(MX, MY)

## A group id vector.
G = c(array(1, 1000), array(2, 1000))

## Generate side by side boxplots.
boxplot(M ~ G, names=c('First instrument', 'Second instrument'))