In this answer I suggested that "the majority" of NPCs would be 0-level characters, and was told by our friendly moderator @BrianBallson-Stanton in a comment (since deleted) that D&D 4 didn't have (or wasn't supposed to have) level gaps.
In AD&D (2nd Edition) campaigns that I ran and played in with my friends, in the absence of any written canon guidelines, we made assumptions about the relative levels of the NPC population of the worlds that we called the "Fifty-Percent Rule"; that the majority (i.e. 50%) would be level 0 characters, and of the other 50%, 50% of them would be Level 1, and so on, so that:
Level 0 : 50%
Level 1 : 25%
Level 2 : 12.5%
Level 3 : 6.25%
Level 4 : 3.125%
Level 5 : 1.5625%
Level 6 : 0.78125%
Level 7 : 0.390625%
Level 8 : 0.1953125%
Level 9 : 0.09765625%
Level 10: 0.048828125%
Level 11: 0.0244140625%
Level 12: 0.01220703125%
Level 13: 0.006103515625%
Level 14: 0.0030517578125%
Level 15: 0.00152587890625%
Level 16: 0.000762939453125%
Level 17: 0.0003814697265625%
Level 18: 0.00019073486328125%
Level 19: 0.000095367431640625%
Level 20: 0.0000476837158203125%
and so on...
Referring to the answer I gave, for a D&D 4e caster to be able to cast Raise Dead, a Level 8 spell, they would need to be 8th level at minimum, i.e. in the top 0.390625% of the population. In a city with a population of 10,000 (Not unreasonable for a D&D-type world), this would be 39 individuals of any character type of Level 8+. Given that most of them (say, 90%) would be unable to cast the spell in question, there may be at most three or four (3.9) who could actually Raise Dead.
My question is:
Is this a reasonable breakdown of the population distribution of experience levels in a D&D campaign (any version)? Is there some canon reference in any version of D&D that states an actual level-population distribution?
Best Answer
Not for modern editions. It varies by edition assumptions and by the campaign tone, furthermore, if you're looking for a natural distribution, you should be using the power-law distribution instead.
At the end of the day, one of the most critical questions in a D&D campaign is "how high magic is this?" And, if the world-designer is being methodical about it, it boils down to a question of "how many high-powered individuals are there in the world?"
Changing the ratio makes for a very different "seeming" world, as for the earlier editions, the narrative of the world changed with power.
In 4e, my experience is that the system doesn't care about simulating the world. The narrative focuses on antagonists and challenges appropriate to the party level. Thus, skill checks scale as the narrative scales, which means that the narrative dictates if there are casters able to perform certain rituals. With that said, Dragon 397 with its hireling rules certainly suggests that a party can get hirelings of their level, and scales costs appropriately.
If you're looking for a naturalistic representation of XP, assuming anyone can earn XP, level distributions should follow a power-law curve.