function stdev( arr, j ) {
	var delta;
	var mean;
	var M2;
	var i;
	var x;
	delta = 0.0;
	mean = 0.0;
	M2 = 0.0;

	for ( i = 0; i < arr.shape[0]; i++ ) {
		x = arr.get( i, j );
		delta = x - mean;
		mean += delta / ( i+1 );
		M2 += delta * ( x - mean );
	}
	return sqrt( M2 / ( i - 1 ) );
}
Computes the unbiased standard deviation.

@private
@param {ndarrayLike} arr - input array
@param {number} j - column for which to calculate the standard deviation
@returns {number} standard deviation

@example
var ndarrayLike = require( './ndarray_like.js' );

var x = [ 0.6333, 0.8643, 1.0952, 1.3262, 1.5571, 1.7881, 2.019, 2.25, 2.481, 2.7119 ];
var y = [ -0.0468, 0.8012, 1.6492, 2.4973, 3.3454, 4.1934, 5.0415, 5.8896, 6.7376, 7.5857 ];
var arr = ndarrayLike( x, y );
var out = stdev( arr, 1 );
// returns ~2.568