  An approximate formula has been derived for gain fluctuations in cascaded
gaseous detectors such as GEM-s, based on the assumption that the charge
collection, avalanche formation and extraction steps are independent cascaded
processes. In order to test the approximation experimentally, a setup involving
a standard GEM layer has been constructed to measure the energy resolution for
5.9 keV gamma particles. The formula reasonably traces both the charge
collection as well as the extraction process dependence of the energy
resolution. Such analytic approximation for gain fluctuations can be applied to
multi-GEM detectors where it aids the interpretation of measurements as well as
simulations.
