That formula was developed by Aladin Akyurek and Harlan Grove.
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In the COUNTIF(A1:A10,A1:A10&"") section, this part: A1:A10&"" ensures that the COUNTIF function will always return at least 1 and never 0 (which would cause the formula to error out when the division is performed.)
Consequently, the numerator for blank cells is 0, so they are not counted.
The numerator for all non-blanks is 1.
The denominator for all non-blanks is their count.
If a value appears 3 times, three of the numerators will be 1 and their respective denominators will be 3.
The SUMPRODUCT function will add those
three fractions: (1/3)+(1/3)+(1/3)=1
That's how the three occurrences only count as a single instance of a unique value.
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