re: Knotty pine

I have seen good and bad knots. They can strengthen or weaken a board. What I am referring to here tho is knotty white pine with the knot passing perpendicular through the wood. As well if the knot is cut thru on the edge of a board the knot itself has a tendency to pop out of the wood. But for argument sake lets just say that the knot is stable and encircled with regular grain wood. In knotty pine the knots will tend to weaken not strengthen the board. The question remains by how much is it weakened?