Typically when measuring intensity, we make a series of measurements. From this we determine the mean intensity and its standard deviation. But how many measurements should we make? Time is often too limited or too expensive to make many long measurements. Therefore should we make many short measurements or only a few long ones?

The mean `*x* and standard deviation *s* of a series of measurements are given by

where *x _{i}* are the individual measurements
and

Often it is not recognised that this estimate of standard deviation is not precise, especially when only a small numbers of measurements are made. The real standard deviation s is given by

where *c ^{2}*[

The length of an intensity measurement is commonly called the integration time. The total measurement time therefore is the product of the integration time and the number of measurements.