function interquartileRange(x) {
    // Interquartile range is the span between the upper quartile,
    // at `0.75`, and lower quartile, `0.25`
    const q1 = quantile(x, 0.75);
    const q2 = quantile(x, 0.25);

    if (typeof q1 === "number" && typeof q2 === "number") {
        return q1 - q2;
    }
}
The [Interquartile range](http://en.wikipedia.org/wiki/Interquartile_range) is
a measure of statistical dispersion, or how scattered, spread, or
concentrated a distribution is. It's computed as the difference between
the third quartile and first quartile.

@param {Array<number>} x sample of one or more numbers
@returns {number} interquartile range: the span between lower and upper quartile,
0.25 and 0.75
@example
interquartileRange([0, 1, 2, 3]); // => 2