function qtrngo( p, v, r ) {
	var q;

	var t = apnorminv( 0.5 + ( 0.5 * p ) );
	if ( v < VMAX ) {
		t += ( ( t * t * t ) + t) / v / 4.0;
	}
	q = c1 - ( c2 * t );
	if ( v < VMAX ) {
		q -= ( c3 / v ) + ( c4 * t / v );
	}
	return t * ( ( q * ln(r - 1.0) ) + c5 );
}
Calculates an initial percentile from the studentized range distribution.

@private
@param {number} p - quantile
@param {number} v - degrees of freedom
@param {number} r - number of samples
@returns {number} initial percentile