docs: function reduce() {
  		var delta = 0;
  		/**
  		* FUNCTION: reduce( acc, data )
  		*	Defines the data reduction.
  		*
  		* @private
  		* @param {object} acc - accumulation object containing three properties: N, mean, sos. 'N' is the observation number accumulator; 'mean' is the mean accumulator; 'sos' is the sum of squared differences accumulator
  		* @param {number} data - numeric stream data
  		* @returns {object} accumulation object
  		*/
  		return function reduce( acc, x ) {
  			// SOS = Sum of Squared Differences
  			acc.N += 1;
  			delta = x - acc.mean;
  			acc.mean += delta / acc.N;
  			acc.sos += delta * ( x-acc.mean );
  			return acc;
  		};
  	}