Even simpler statistics

SimpleStats.js

function SimpleStats(optionalInputArray) {
	var self = this; 
	var setSize = 0, 
	    arithmeticMean = 0, 
	    standardDeviation = 0, 
	    pwrSumAvg = 0;
	
	//http://en.wikipedia.org/wiki/Mean#Arithmetic_mean_.28AM.29
	var setArithmeticMean = function(x) {

        arithmeticMean += (x - arithmeticMean) / setSize;

	}

	
	//http://en.wikipedia.org/wiki/Standard_deviation
	var setStandardDeviation = function(x) {
        pwrSumAvg += ( x * x - pwrSumAvg) / setSize;
        standardDeviation = Math.pow( (pwrSumAvg * setSize - arithmeticMean * arithmeticMean * setSize) / setSize, 0.5 );
	}	
	
	self.addValues = function(arr) {

        var arrLength = arr.length;

	    for(var i=0;i<arrLength;i++) {
	        self.addValue(arr[i]);
	    }
	    return self; 
	}
	
	self.addValue = function(x) {

	    setSize += 1;
		setArithmeticMean(x); 
		setStandardDeviation(x);
		return self; 
	}
	
	self.getArithmeticMean = function() {
	    return arithmeticMean;
	}
	
	self.getStandardDeviation = function() {
	    return standardDeviation;
	}
	
	if(typeof optionalInputArray != 'undefined') {
	    self.addValues(optionalInputArray);
	}
	
	return self; 
}