Captain Anorak's Guide to Gaming
Consistency: Character Stat Levels
It seems remarkable to me that so few roleplaying games give a clear indication of what level of ability is represented by any given number. Take Call of Cthulhu for example: most of the primary stats are rolled on 3D6, but Intelligence is rolled on 2D6+6. Interpretations of why this is vary, falling into two main camps:

(A) Values of 8, 13 and 18 are the minimum, typical and maximum values for human intelligence, and so rolling 2D6+6 generates characters with the usual distribution of intelligence levels.
(B) Values of 3, 10-11 and 18 are the minimum, typical and maximum values for human intelligence, and so rolling 3D6 would generate characters with the usual distribution of intelligence levels; 2D6+6 in character creation generates PCs of unusually high intelligence.

There is no guidance on this in the rules; as far as I know there never has been, and probably the writers of the game themselves have no clear idea.

When I write a set of rules, one of the first things I do is to write down what stat numbers reflect what level of ability. For primary stats I write something like this:

Stat level number	Stat level word-name	Proportion of the population having that ability level
0	Totally unable	0.01%
1	Feeble	1%
2	Poor	4%
3	Below average	25%
4	Average	40%
5	Above average	25%
6	Good	4%
7	Excellent	1%
8	Excellent	0.1%
9	Excellent	0.01%
10	Excellent	0.001%

Really I find it incredible that most game writers don't bother with this kind of exercise. I don't see how you can write a game without a clear idea of what a stat level represents, and if you do have such a clear idea then it seems an oversight of vast proportions not to make this information clear to the players.

CONSISTENCY ACROSS SOCIETIES

It's particularly important to establish these numbers if you want the ability levels of characters (player and non-player) in your game to be consistent with each other. It's a good idea to map out the typical fighting abilities of various groups. For instance, in a swords-and-sorcery game, it would be sensible for the GM to stat types of people such as the following:
- Typical peasant farmer.
- Typical criminal thug.
- Typical town guardsman.
- Typical tribal warrior.
- Typical professional soldier.
- Typical elite professional soldier.
- Typical member of the Emperor's personal guard.

Once this has been done, it should be easy to see where any individual should fall in the range of fighting skill levels.

Stat Change over Time.