function pdf( x ) {
		var ez;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return 0.0;
		}
		z = abs( ( x - mu ) / s );
		ez = exp( -z );
		return ez / ( s * pow( 1.0 + ez, 2.0 ) );
	}
Evaluates the probability density function (PDF) for a logistic distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF

@example
var y = pdf( -1.2 );
// returns <number>