It was found that, the intensity of many radio sources vary as a function of time with time scales ranging from days to years. The most prominent time varying radio emitters are pulsars. Pulsars are the objects whose radio signals switched on and off at rapid regular intervals, known as period of the pulsar. So by observing the radio source for longer period one can get information of the variability of the source.
(Ref : Verschuur, `The Invisible Universe', Heidelberg Science Library, 1974.
An Introduction to Radio Astronomy, Luke Woolfenden.
http://www.astro.cardiff.ac.uk/observatory/radioback.html)