What is the 90th percentile and how is it calculated

The 90th percentile is the value for which 90% of the data points are smaller

The 90th percentile is a measure of statistical distribution, not unlike the median. The median is the middle value. The median is the value for which 50% of the values were bigger, and 50% smaller. The 90th percentile tells you the value for which 90% of the data points are smaller and 10% are bigger.

Statistically, to calculate the 90th percentile value:
1. Sort the transaction instances by their value.
2. Remove the top 10% instances.
3. The highest value left is the 90th percentile.

Example:
There are ten instances of transaction "t1" with the values 1,3,2,4,5,20,7,8,9,6 (in sec).
1. Sort by value — 1,2,3,4,5,6,7,8,9,20.
2. Remove top 10 % — remove the value "20."
3. The highest value left is the 90th percentile — 9 is the 90th percentile value.

The 90th percentile value answers the question, "What percentage of my transactions have a response time less than or equal to the 90th percentile value?" Given the above information, here is how LoadRunner calculates the 90th percentile.

In Analysis 6.5:
The values for the transaction are ordered in a list.
The 90% is taken from the ordered list of values. The place from which it is taken is

Rounding to the small value the number: 0.9 * (Number of Values – 1) + 1

In Analysis 7 and above:
Each value is counted in a range of values. For example, 5 can be counted in a range of 4.95 to 5.05, 7.2 in a range of 7.15 to 7.25. The 90% is taken from the range of values that the number of transaction in it and before it is >= ( 0.9 * Number of Values).

This difference in the methods can lead to different 90% values. Again, both methods lead to correct values as defined by the 90th percentile. However, the algorithm to calculate these figures has changed in LoadRunner 7 and above.

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5 thoughts on “What is the 90th percentile and how is it calculated

  1. Frank Cohen

    Thanks for explaining this. What business question does a 90th percentile report answer? It seems like it is a filter to be added to reports on transaction times, step times, and latency measurements. Like I just need to strip out the 10% worst performers to see where is the majority (90%) of the transaction performance. How does 90th percentile help?

    -Frank

    Reply
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