Array.prototype.sum = function() {
    
    return this.myreduce(function(a, b) { return a + b });
}

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

    var sorted = this.slice().sort(function(a,b){return a > b})
    len =sorted.length

    if(len %2)
    return sorted[(len-1)/2]
    
    else
    return sorted.slice((len/2)-1, (len/2) + 1).mean()
    
}

Array.prototype.sumSquared = function() {
    return this.myreduce(function(a, b) { return a + b * b });
}

Array.prototype.variance = function() {
    var mean = this.mean();
    if (mean) {
        var sum = this.sum();
        return Math.abs((this.length * this.sumSquared() - sum * sum) / (this.length * (this.length - 1)));
    } else {
        return Number.NaN;
    }
}

Array.prototype.standardDeviation = function() {
    return Math.sqrt(this.variance());
}


if (!Array.prototype.myreduce)
{
  Array.prototype.myreduce = function(fun /*, initial*/)
  {
    var len = this.length;
    if (typeof fun != "function")
      throw new TypeError();

    // no value to return if no initial value and an empty array
    if (len == 0 && arguments.length == 1)
      return 0;
      //throw new TypeError();

    var i = 0;
    if (arguments.length >= 2)
    {
      var rv = arguments[1];
    }
    else
    {
      do
      {
        if (i in this)
        {
          rv = this[i++];
          break;
        }

        // if array contains no values, no initial value to return
        if (++i >= len)
          throw new TypeError();
      }
      while (true);
    }

    for (; i < len; i++)
    {
      if (i in this)
        rv = fun.call(null, rv, this[i], i, this);
    }

    return rv;
  };
}