code: function _stdNormDensity(x)
  {
    return Math.pow(Math.E, -1 * Math.pow(x, 2) / 2) / Math.sqrt(2 * Math.PI);
  }
docs: Standard normal density function.
  
  @private
  @param {Number} x The value to calculate the standard normal density of
  @returns {Number} The value of the standard normal density function at x