Stat boosts work in 6 stages. if it rises or falls, it goes up/down one stage. if it rise/falls Sharply/Harshly, that means it was a 2 stage change. it works like this:
+1 = 1.5x original stat
+2 = 2x original stat
+3 = 2.5x original stat
+4 = 3x original stat
+5 = 3.5x original stat
+6 = 4x original stat.
So if a pikachu has a base speed of 100, after an Agility, it goes up 2 stages and doubles to become 200. another agility would make it 300, and so on. hope that helps