function variancetk( N, correction, x, stride ) {
	var S2;
	var ix;
	var S;
	var v;
	var n;
	var i;

	n = N - correction;
	if ( N <= 0 || n <= 0.0 ) {
		return NaN;
	}
	if ( N === 1 || stride === 0 ) {
		return 0.0;
	}
	if ( stride < 0 ) {
		ix = (1-N) * stride;
	} else {
		ix = 0;
	}
	S2 = 0.0;
	S = 0.0;
	for ( i = 0; i < N; i++ ) {
		v = x[ ix ];
		S2 += v * v;
		S += v;
		ix += stride;
	}
	return (S2 - ((S/N)*S)) / n;
}
Computes the variance of a strided array using a one-pass textbook algorithm.

@param {PositiveInteger} N - number of indexed elements
@param {number} correction - degrees of freedom adjustment
@param {NumericArray} x - input array
@param {integer} stride - stride length
@returns {number} variance

@example
var x = [ 1.0, -2.0, 2.0 ];

var v = variancetk( x.length, 1, x, 1 );
// returns ~4.3333