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

where *x*_{i} are the individual measurements
and *n* is the number of measurements. The standard deviation is an
estimate of the spread in the measurements (population). Strictly this is valid only if the
variations in *x*_{i} about the mean follow a normal distribution.

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}[*a*;*b*] is an inverse form of the *c *^{2} distribution, called the percentage points of the *c *^{2} distribution, and is the value of *c *^{2} which would give a *c *^{2} distribution value of *a* for *b* degrees of freedom; *a* is the confidence level. If *a* =0.9 then there is a 90% confidence that s is inside the two limits given above.

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.