function factory( mu, s ) {
	var ls;
	if ( isnan( mu ) || isnan( s ) || s < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( s === 0.0 ) {
		return degenerate( mu );
	}
	ls = ln( s );
	return logpdf;

	/**
	* Evaluates the logarithm of the probability density function (PDF) for a logistic distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPDF
	*
	* @example
	* var y = logpdf( -1.2 );
	* // returns <number>
	*/
	function logpdf( x ) {
		var az;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return NINF;
		}
		z = ( x - mu ) / s;
		az = -abs( z );
		return az - (2.0 * log1p( exp( az ) )) - ls;
	}
}
Returns a function for evaluating the logarithm of the probability density function (PDF) for a logistic distribution.

@param {number} mu - location parameter
@param {NonNegativeNumber} s - scale parameter
@returns {Function} logPDF

@example
var logpdf = factory( 10.0, 2.0 );
var y = logpdf( 10.0 );
// returns ~-2.079

y = logpdf( 5.0 );
// returns ~-3.351