"Yes," "yes," and "YES" are counting separately in my filter — how to standardize
Yes, yes, and YES showing up as separate groups in a filter or count? Standard Excel and Sheets comparisons are actually case-insensitive, so the real culprit is usually a case-sensitive tool in the chain. Here's how to standardize the values
The part that's counter-intuitive: Excel isn't case-sensitive by default
Most people assume "Yes" and "yes" are different to a computer, but Excel's core text comparison, the plain equals sign, treats letter case as irrelevant. COUNTIF("Yes") counts cells containing yes, Yes, and YES all together, and FILTER's default equality check works the same way. So if your results are splitting by case, the ordinary comparison isn't what's doing it.
Where case-sensitivity actually sneaks in
Power Query is the most common source — its Group By and Remove Duplicates transformations compare text case-sensitively, unlike Excel's native formulas, so a query built on inconsistently-cased data will produce separate groups even though a plain COUNTIF on the same column wouldn't. The EXACT() function is another route in, since it's specifically built to be case-sensitive and some more elaborate FILTER or SUMPRODUCT formulas use it on purpose without the author realizing the side effect. In Google Sheets, the QUERY() function's SQL-style WHERE clause on text is also case-sensitive, unlike a plain FILTER condition.
Standardizing the values so this can't happen again
Add a helper column with =UPPER(A2) or =LOWER(A2) and build your grouping or filtering logic on that normalized column instead of the raw one — this sidesteps the issue regardless of which case-sensitive tool is causing it. If the source data itself should be consistent, Find and Replace can standardize it directly, and a Data Validation dropdown list on the entry field prevents new case variants from being typed in going forward.