In statistics, the Fano factor,[1] like the coefficient of variation, is a measure of the dispersion of a counting process. It was originally used to measure the Fano noise in ion detectors. It is named after Ugo Fano, an Italian-American physicist.
The Fano factor after a time is defined as
where is the standard deviation and is the mean number of events of a counting process after some time . The Fano factor can be viewed as a kind of noise-to-signal ratio; it is a measure of the reliability with which the waiting time random variable can be estimated after several random events.
For a Poisson counting process, the variance in the count equals the mean count, so .