What's the exact mathematical formula of calculating “top n% of this X”?

After getting the answer of this question, I am confused. It says: `n%` is percentage of users having reputation change this X.

Its alright, but how does this no. get smaller for top rankers? If it has to do with user base below it too.. `n%` should be higher with simple maths.

I want to know exact formula system uses to calculate it. My concern is `top n%`, not `this X`. I know, `this X` is chosen based on best performance among all X.

• "Its alright, but how does this no. get smaller for top rankers? If it has to do with user base below it too.. n% should be higher with simple maths." Can you explain that a bit? I don't understand this paragraph at all. – sepp2k Mar 23 '12 at 10:20
• It's a percent, which means by definition yes, it depends on how many other users are involved. The only way the percentage gets smaller is if more items are involved, nothing crazy is happening here it's just a very standard percentile rank. Can you be more specific about which part isn't clear? – Nick Craver Mar 23 '12 at 10:43
• @sepp2k suppose all users are 100 & users with reputation change are 51. For 50th one, 1 user is below him. For 1st one, 50 users are below him. Percentage of 50 (out of 100) is greater or Percentage of 1? This is what I was saying. But, looks like Juhana has addressed this issue in answer. – user178465 Mar 23 '12 at 12:01
• @NickCraver I think, its percentage, not percentile no. And, check my previous comment for what I want to say in question. – user178465 Mar 23 '12 at 12:04
• @NickCraver From the answer, it looks like its really percentile no. But, `%` symbol is confusing me.. – user178465 Mar 23 '12 at 14:37

• Put all users in a list, ordered by reputation. The formula is `x / n` where `x` is the position in the list and `n` is the total amount of users (with reputation gain, I assume). – JJJ Mar 23 '12 at 11:56