code: function cdf( x, mu, s ) {
  	var z = ( x - mu ) / s;
  	return 1 / ( 1 + exp( -z ) );
  }
docs: CDF // 
  FUNCTION: cdf( x, mu, s )
  Evaluates the cumulative distribution function (CDF) for a Logistic distribution with location parameter `mu` and scale parameter `s` at a value `x`.
  
  @param {Number} x - input value
  @param {Number} mu - location parameter
  @param {Number} s - scale parameter
  @returns {Number} evaluated CDF