If your pivot table defaults to Count instead of Sum, then you have inconsistent numerical data.
I found this out on a webinar about five years ago, and it still seems to be a mostly unknown feature.
And yes, I call it a feature, not a bug, because it is ultimately helping you.
What it is doing is giving you an immediate message that your numerical data has gaps, or perhaps has numbers formatted as text, or other errors. If you ignore this, you might produce inaccurate analysis.
So – if your pivot table defaults to Count, don’t do what I used to do, which was to tut and manually change it to Sum.
Investigate your data carefully until you’ve found the inconsistencies and then try it again until it defaults to Sum. Here’s a link to some suggestions for converting text to numbers.
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