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.
Did anyone else already saw a similar issue?
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.
This issue has been mentioned on Matomo forums. There might be relevant details there:
@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?
@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!)
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.
I think that on the same page (see Chardonneaur screenshot), you should not have 2 different values for the same thing:
All values are the same, except Total...