I used to worry if you should include counting error
in N in error estimate which otherwise only involves the Standard
Deviation of f.
If N is a priori fixed, clearly there is no error
but suppose the events used in Equation (26) are those that fall in a
particular subregion (counter) of a bigger sample space (and is
defined only over this subregion).
Then the estimate of probability of falling into counter
certainly involves error! What about the mean of a quantity
within subregion---does this involve this error?
The problem is: what is error in a moment: should it include
counting error?
The answer is no---one uses identical formulae in
case where number of events prescribed and not subject to counting
error.