I was looking at a bug report again: https://bugs.documentfoundation.org/show_bug.cgi?id=132983
The reporter is scratching their head over the "division by zero" result for z.
From what I can see, the "Known variance" values can't both be zero, if you want to avoid the #DIV/0! results. You have to give at least one of them a positive non-zero value.
Now, looking at Help:
https://help.libreoffice.org/7.1/en-US/text/scalc/01/statistics_test_z.html
and the Calc guide chapter CG7009-DataAnalysis-SF-12Aug2020, we see this same "division by zero" result and the reader is left holding it.
How to present this better in the docs? I have no experience on the topic, but I hope we can find some statistician to shed light on this!
Ilmari