function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a || x > b ) {
			return 0.0;
		}
		return 1.0 / ( b - a );
	}
Evaluates the probability density function (PDF) for a uniform distribution.

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

@example
var y = pdf( 2.0 );
// returns <number>