How to add a standard deviation metric

[psicomante]

Hello,

k6 is a wonderful tool, thank you.

I wonder if is it possible to add a custom metric that calculates the standard deviation of some http time. I guess it's not possible to do it via Javascript bindings, am i correct?

Thank you.

[robingustafsson]

Hi @psicomante, pardon the late reply. You're correct, it's currently not supported directly through the k6/metrics API (custom metrics), however you could output the result to JSON [1] and then use jq to query the data [2] and calculate the standard deviation [3]. You could either filter on the URL tag, create a custom tag for the requests you're interested in, or set up a custom metric (Trend) to track the response times that you could then filter on using jq.

[1] - https://docs.k6.io/docs/results-output#section-json-output
[2] - https://docs.k6.io/docs/results-output#section-processing-json-output
[3] - https://stackoverflow.com/questions/15101343/standard-deviation-of-an-arbitrary-number-of-numbers-using-bc-or-other-standard

[na--]

This should probably be implemented after we start using HDR histograms - #763
And it still should be something that's only calculated on demand, since it iterates over all of the metric values, but maybe as part of receiving the summary metrics at the end of the test (#351)?

[john-wd]

Is this suggestion still under the consideration from the k6 team?