When Excel’s SUM is wrong …

Or why data analytics depends on data preparation.

I wanted to find the “Reach” of some Facebook groups. In my mind, it should have been a simple SUM calculation. It should have been an easy formula using Excel’s Sum Function. We can add all the values of “# of Members” up to get a total. So I open my staff’s FB groups list in XLSX format. And boy was I wrong!

Column A contains the FBgroup name (anonymized) and Column B contains the number of members for each group. This is shown below: