Possible Duplicate:
Is it better to spawn your first overlord at 9 or 10 supply?
As Zerg, I can make an overlord after my 9th drone or after my 10th. If I make it after the 10th, I can build an extrator, a 11th drone then cancel the extractor.
Does anyone have concrete facts comparing the 2 strategies? Is one better than the other? It doesn't have to be a large gap, just "This one is better by X minerals".