The stats raise and lower in 6 stages each. Some moves raise the stat by one stage, but ones where it says "sharply increased" raise it by two stages.
The stages are: x1.5, x2.0, x2.5, x3.0, x3.5, x4.0. So, for example using Sword Dance which sharply increases Attack, will double your attack stat the first time. The second time it will be 3 times the original value.
The same stages apply for lowering stats, you just divide by those numbers instead. Two stages down and the stat is half of what it was.