stats.js

/*
 * Define some useful statistical functions on arrays of numbers
 */

Array.prototype.sum = function() {
    var i, sum = 0;
    for (i = 0; i < this.length; i++) {
        sum += this[i];
    }
    return sum;
}

Array.prototype.max = function() {
    return Math.max.apply(null, this);
}

Array.prototype.min = function() {
    return Math.min.apply(null, this);
}

Array.prototype.mean = function() {
    return this.sum() / this.length;
}
Array.prototype.average = Array.prototype.mean;

Array.prototype.median = function() {
    var sorted = this.sort( function(a,b) { return a-b; }),
        len = sorted.length;
    if (len % 2) {
        return sorted[Math.floor(len / 2)]; // Odd
    } else {
        return (sorted[len/2 - 1] + sorted[len/2]) / 2; // Even
    }
}

Array.prototype.stdDev = function(sample) {
    var i, sumSqr = 0, mean = this.mean(), N;

    if (sample) {
        // Population correction if this is a sample
        N = this.length - 1;
    } else {
        // Standard deviation of just the array
        N = this.length;
    }

    for (i = 0; i < this.length; i++) {
        sumSqr += Math.pow(this[i] - mean, 2);
    }

    return Math.sqrt(sumSqr / N);
}