I get this asked a lot and if I tell people "If Carmack and Torvalds are a 10 and someone who can write a hello world is a 1, then I would rate myself 5 max" and most just think I must be a bad programmer because of that rating.
Well even with those bounds, is that scale in terms of the population as a whole, some measurable notion of productivity, or in terms of some abstract concept of absolute skill? Depending on the definition, 5 could mean many different things.
Not to mention that there are different aspects of skill. Is Torvalds a good functional programmer? (I actually have no idea, so this might be a bad example, but hopefully I make my point.)