Average of averages is not the average of initial values
Be careful with aggregating of aggregated. Sum of sum is sum of initial values, but not the average of averages.
As groups have different sizes, if you do average their averages you calculate them equal-weighted, ignore their size.
Calculate overall quality/ average as you calculate it for each individual grooup and not averge the averages!
You can use calculated field in pivot (Sum/ Num) to calculate the average or ( sum of good/ total sum ) to quality.
Using the type of calculation-average in pivot will be mistake ...
How I learned
Again, angry calls. :) .
We have to calculate quality parameters for each department and to each district. And again the average quality of district is not the average of quality of its departments. As they are not equall....