The formula for finding Percentile rank as per wikipedia is
$$\frac{c_{\ell}+0.5 f_{i}}{N} \times 100 \%$$
One of the edge cases in my application is when all the values are zero. In this case the number of scores less than the score of interest is zero, the frequency is equal to the number of total values which is $N$.
This formula gives me a rank percentile of $50\%$ when all the scores are equal and zero. It seems counter intuitive to suggest that the rank percentile is $50\%$ even though there is not a single score less or more than the score of interest. Is this the correct value?
Best Answer
Other possible values would be 0% (if by rank percentile you mean the percent strictly less than) or 100% (if you mean the percent less than or equal to). But ranking is pretty meaningless when all the values are the same, so 50% is as correct as either of the above.