function accumulator( f, a ) {
		if ( arguments.length === 0 ) {
			return mean();
		}
		return mean( 100.0 * ((a-f)/a) );
	}
If provided input values, the accumulator function returns an updated mean percentage error. If not provided input values, the accumulator function returns the current mean percentage error.

@private
@param {number} [f] - input value
@param {number} [a] - input value
@returns {(number|null)} mean percentage error or null