1. ## 90th Percentile

Hello,

I am in search of the exact formula for calculating 90th percentile value.

Load Runner is using some sorting algos for this. But I am using SilkPerformer and it was using a different formula. Microsoft Excel Function also following that formula (i am getting the same values from excel &amp; SP).

Regards,
N.Karthikeyan.

2. ## Re: 90th Percentile

Hi,

Here is the link from wikipedia on Percentiles.

Percentile - Wikipedia

It seems there are differents versions to calculate centiles (see the External Links and the Alternate Versions).

Hope it helps,

Regards,

Robin

3. ## Re: 90th Percentile

The algorithm I use for generating percentiles is this:

- populate an array containing your numbers
- sort the array in ascending order
- divide the number of elements in the array by the percentile you want (round down to the nearest integer to get N)
- the Nth element in your array will be the value of that percentile

Now I'm curious what other tools are using to calculate percentiles.

4. ## Re: 90th Percentile

Thanks for your replies dudes! You helped me a lot!

5. ## Re: 90th Percentile

I went through that things and got the formula which excel is using. But i couldn't get the formula used by the SilkPerformer

6. ## Re: 90th Percentile

Hello,

If you have all the values, it is easy to calculate 90h percentile value: you need to sort your values in the ascending order and take the value direct as Corey Goldberg mentioned above.

In case of load testing tools it is likely that all the values can not be saved, because it would require too much RAM. That is why such tools have to use some approximation to calculate this parameter. For example, the following standard deviation formula can be used: http://www.medcalc.be/manual/summary_statistics.php. However this formula can only be used in cases when the distribution of the values is close to the normal distribution.

7. ## Re: 90th Percentile

Thanks a lot!

8. ## Re: 90th Percentile

[ QUOTE ]
That is why such tools have to use some approximation to calculate this parameter.

[/ QUOTE ]

the link you posted didn't work.
I am very interested in seeing how to approximate percentiles without storing all of the values during test execution.. any more explanation or links?

9. ## Re: 90th Percentile

[ QUOTE ]
the link you posted didn't work.

[/ QUOTE ]

Take off the trailing period.

http://www.medcalc.be/manual/summary_statistics.php

10. ## Re: 90th Percentile

(thanks Joe)

Softlogica,
thanks for the info. i think I understand the concept now, but you are right, the distribution has to be VERY close to Normal for it to work. Skewed distributions wouldn't work.

interesting stuff.
I think more performance testing tools should use this technique.

I have written my own performance tool.. it does percentiles, but it carries all of the data in memory, which won't work for massive tests.

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