public function PI($x, $p, $q = 1)
{
$V = $this->regressionVariance($x) + 1 / $q;
$σ² = $this->meanSquareResidual();
// The t-value
$t = StudentT::inverse2Tails($p, $this->ν);
return $t * sqrt($σ² * $V);
}