function variance(x) {
    if (x.length === 0) {
        throw new Error("variance requires at least one data point");
    }

    // Find the mean of squared deviations between the
    // mean value and each value.
    return sumNthPowerDeviations(x, 2) / x.length;
}
The [variance](http://en.wikipedia.org/wiki/Variance)
is the sum of squared deviations from the mean.

This is an implementation of variance, not sample variance:
see the `sampleVariance` method if you want a sample measure.

@param {Array<number>} x a population of one or more data points
@returns {number} variance: a value greater than or equal to zero.
zero indicates that all values are identical.
@throws {Error} if x's length is 0
@example
variance([1, 2, 3, 4, 5, 6]); // => 2.9166666666666665