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Discrepancies noticed within the performance report #19729

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Chardonneaur opened this issue Sep 13, 2022 · 8 comments
Open

Discrepancies noticed within the performance report #19729

Chardonneaur opened this issue Sep 13, 2022 · 8 comments
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Bug For errors / faults / flaws.

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@Chardonneaur
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Hi guys,

A client of mine reported me the following issue, the total of the performance report for a given month on the bar graph is different than the one of the table shown below it.
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Did anyone else already saw a similar issue?

@Chardonneaur Chardonneaur added the Potential Bug Something that might be a bug, but can't be reproduced (yet). label Sep 13, 2022
@sgiehl
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sgiehl commented Sep 13, 2022

Hey @Chardonneaur
This could maybe be rounding issues. The bar evolution chart simply displays a sum of all average metrics, while the average load time metric is calculated on a per visit base, summed up and divided through the number of visits.

@MatomoForumNotifications

This issue has been mentioned on Matomo forums. There might be relevant details there:

https://forum.matomo.org/t/average-page-load-time-incoherence/47384/2

@Chardonneaur
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@sgiehl I tried to test the rounding hypothesis and to me it is not because the difference is too high.
Is there any possibility to check the calculation that Matomo is doing to perform the total of this table report?

@heurteph-ei
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@sgiehl, I agree @Chardonneaur, even with rounding, you can't have such difference.
On the tooltip, the rounding is acceptable (and not visible): 0.2 + 1.0 + 0.08+0.93+1.09+0=3.30, on the KPI, it is not. with approximations, the maximum gap should be 0.06 (in the screenshot the difference is 0.51s, or 15%, what a huge difference!)

@sgiehl
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sgiehl commented Oct 12, 2022

Those are some sort of rounding and calculation issues.
Each single performance metric is calculated by using the sum over all actions, having the metric set, divided through the count of actions.
But the overall metric is computed by summing all metrics over all actions, where one of those metrics have a value, divided through the actions.
So in theory there can be actions, that e.g. only have a transport time set. Those would then also be used for the overall metric, but would lower it's value bit more, as other metrics aren't set.
Hope that makes sense. If not I might need to explain that a bit more in detail.

@heurteph-ei
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I think that on the same page (see Chardonneaur screenshot), you should not have 2 different values for the same thing:
image
image
All values are the same, except Total...

@Chardonneaur
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@sgiehl @heurteph-ei thank you very much for taking the time investing in this issue, I think it is really important that an answer can be provided to the community in an FAQ or similar as the question will come on and on.

@sgiehl
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sgiehl commented Oct 14, 2022

@heurteph-ei @Chardonneaur I agree that there shouldn't be two different values. I guess we actually should change the calculation of the value used for the sparkline. Otherwise it might be a bit strange that the total page load time isn't the sum of all other metrics. 🤔

@sgiehl sgiehl added Bug For errors / faults / flaws. and removed Potential Bug Something that might be a bug, but can't be reproduced (yet). labels Oct 14, 2022
@sgiehl sgiehl added this to the For Prioritization milestone Oct 14, 2022
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