Nine Balls
You have nine balls, three of which are slightly heavier, and three of which are slightly lighter than the normal three. All three balls in each group have the same weight. All nine balls are identical otherwise. Your mission is to determine the group of every ball using a pair of scales.

How many weighings are needed in order to ensure the accomplishment of your mission?

Notes:
1. The difference of weight between the heavy and normal balls is not necessarily equal to the difference of weight between the light and normal balls.
2. The scales show only which side is heavier; it does not show the difference of weight.