Hi,

It is likely due to floating-point arithmetic, which can cause small rounding errors in calculations. This is a common issue in computing when dealing with floating-point numbers, where the precision is finite, and calculations can result in numbers that are very close to, but not exactly, zero.

To ensure that the RSD calculation in Excel returns 0 when the standard deviation is smaller than a certain threshold, such as 0.00001, you can use the following formula with an IF statement:

`=IF(STDEV.S(B1:B12)<0.00001, 0, STDEV.S(B1:B12)/AVERAGE(B1:B12))`

This formula will check if the standard deviation of the range B1:B12 is less than 0.00001. If it is, the formula will return 0, which is the expected RSD when all numbers are identical or the variation is negligible. If the standard deviation is not less than 0.00001, it will proceed to calculate the RSD normally. And you can also customize the threshold value on your own.