I have used it many times and without a problem until one day, a tyke asked me, "why is it 99th percentile and not just 99"? I don't know how to explain it to him. Then I thought, jeez, I don't know myself. can anyone explain?

I have found out so many definitions given online but I couldn't connect with any of them. they only had examples of its usage and no account of why it was so. so why is it called 99th percentile?

closed as off-topic by ruakh, RyeɃreḁd, TimLymington, choster, tchrist Apr 29 '14 at 23:17

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  • 1
    You might like the site Mathematics Educators – Matt E. Эллен Apr 27 '14 at 7:46
  • 1
    I'm not exactly clear on what you're asking. Is it why -th? What is a percentile? Because if you know what a percentile is, it's just a matter of identifying them: first %ile, second %ile... 98th %ile, 99th %ile. Like a game: 1st quarter, 2nd..., 3rd..., 4th. – anongoodnurse Apr 27 '14 at 7:51
  • possible duplicate of "Percent" vs. "percentile" – choster Apr 28 '14 at 3:43

A percentile is a grouping of a population or field into 100 equal parts, based on a the metric that is observed. Income is a common metric so that we can say that first percentile is the 1% of the population that earn the least, etc. Other common groupings include quartiles, which is the same concept, but dividing the group into 4 equal parts, and so on.

Having divided up our field into groups, we can see that it doesn't make sense to say "the 1 percentile" or "the 99 percentile", just like it doesn't make sense to say "the 1 quartile" or "the 3 quartile" - we have an ordered series of groupings, and we're referring to the "first percentile" or "the third quartile" - the number 3 or 99 here are referring to the percentile's position in the group, in-order. So we have the first and last and middle and 99th percentile.

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