# Percentile Versus Exceedance .. whats the difference?

• Mar 17th 2013, 01:12 PM
shino
Percentile Versus Exceedance .. whats the difference?
Hi there, I am a student hoping to get into engineering. I havent had much formal math education in the way of probability of statistics, but I've been trying to teach myself using free online resources.

I am hoping all you experts out there can help me with my problem!

I am looking at wind power production for a project, predicted for a 7 year period. I'm trying to construct the MWh value which is exceeded 99% of the time. I am trying to find this value for every hour in every month (12 x 24 table).

To do this using an exceedance interval, P99:

0.99 = x/(days in month +1) solve for x; where x is the 'rank' of that value, in descending order (generally around 28 out of 31), then find the MWh value that corresponds to rank x.

To do this using a percentile: The 99th percentile is the value that is greater than 99% of the dataset. So to turn this around for my purposes, could I say I am looking for the 1st percentile; that is the value that is greater than 1% of the dataset, meaning 99% of the data is greater than this value.

My question is, is all the above valid. Can I say P99 = 1st percentile?

• Mar 17th 2013, 02:19 PM
HallsofIvy
Re: Percentile Versus Exceedance .. whats the difference?
Sorry but I have never seen the term "exeedance". Can you give the definition?
• Mar 17th 2013, 03:42 PM
shino
Re: Percentile Versus Exceedance .. whats the difference?
Quote:

Originally Posted by HallsofIvy
Sorry but I have never seen the term "exeedance". Can you give the definition?

My Apologies, I'm probably goofing up the terminology.

By P99 Exceedance, I mean the MWh Value that is exceeded 99% of the time. Perhaps the term I am looking for is confidence interval.