Note that this function computes the quartiles using the
quantile function rather than following
IQR(x) = quantile(x, 3/4) - quantile(x, 1/4).
For normally \(N(m,1)\) distributed \(X\), the expected value of
2*qnorm(3/4) = 1.3490, i.e., for a normal-consistent
estimate of the standard deviation, use
IQR(x) / 1.349.