It is in levels with a maximum of 6 up or down, but I'm not sure what the level is. If you find out -

*harshly fell* = lowered by 2 levels - Draco meteor's recoil harshly lowers Sp attack
*fell* = lowered by one level- tail whip lowers defence by one level
*rose* = went up by one- defence curl raises defence by one level
*rose sharply* = went up by two- swords dance raises attack by 2 levels
- EDIT- thanks to hex-
*drastically* = went up by three (only applies to 2 moves- tail glow & cotton guard)

The maximum of levels you can lower your opponents or your stats is 6 levels & the maximum you can raise your or your opponents stats is 6 levels.

Simple doubles the effects of stat raising or lowering moves- with simple, tail glow will automatically max the Sp. Attack stat boost ( instead of raising by 3 levels, it raises by 6)

EDIT: see this question for the levels. They are: x1.5, x2.0, x2.5, x3.0, x3.5, x4.0.