function(y, x)
{
	n <- length(y)
	xs <- cbind(jay(n, 1), x)
	b <- inv(t(xs) %*% xs) %*% t(xs) %*% y
	yhat <- xs %*% b
	err <- y - yhat
	ht <- xleverage(x)
	sg <- sqrt(sum((y - yhat)^2)/(n - ncol(xs)))
	sag <- err/(sg * sqrt(1 - ht))
	sag <- matrix(sag, ncol = 1)
	sag
}
