How is the aggregated percentile value calculated in the Response Time Statistics report?

[WillCoutinho]

I couldn’t find an explanation in the documentation about how the aggregated percentile (e.g., P90) is calculated in the Response Time Statistics report.

When I compare each request’s P90 with the aggregated value, they don’t match. For example, in the image, the P90 shows this list of values, but the aggregated value does not match any method I tried (sum, average, or ordering the values and checking the 0.9 percentile).

Is there a method to reproduce the aggregated value manually? Any clarification would help me explain this metric to stakeholders.

Values from image: [1100, 720, 800, 1700, 930, 1500, 720, 650, 710, 6100, 770, 1700, 1200]

[cyberw]

Hi!

If your question is: what are response percentiles, there's a good explanation here: https://stackoverflow.com/a/32869772/1591921

Percentiles are very heavy to calculate in real time for large numbers of requests, so they are all approximated.

When I compare each request’s P90 with the aggregated value, they don’t match

When you say that, what do you mean specifically? How/why do you expect them to "match"? The "aggregated" P90 is the 90th percentile for all requests, not something like an average of the individual request percentiles or anything.

[WillCoutinho]

Percentiles are very heavy to calculate in real time for large numbers of requests, so they are all approximated.

Got it. This answers my question.

When you say that, what do you mean specifically? How/why do you expect them to "match"? The "aggregated" P90 is the 90th percentile for all requests, not something like an average of the individual request percentiles or anything.

I thought the aggregated value in the "90%ile" column was simply the 90th value from the list I marked in red on the image, when ordered in ascending order. But now I understand that the aggregated value comes from all the requests made, not directly from that column.

Also, just to confirm: does this same logic for aggregated values apply to the "Requests Statistics" table as well, in the same way it works for the "Response Time Statistics"?

[cyberw]

I thought the aggregated value in the "90%ile" column was simply the 90th value from the list I marked in red on the image, when ordered in ascending order. But now I understand that the aggregated value comes from all the requests made, not directly from that column.

Exactly, its not the 90th percentile of the 90th percentile or anything :)

Also, just to confirm: does this same logic for aggregated values apply to the "Requests Statistics" table as well, in the same way it works for the "Response Time Statistics"?

Yes.