We report on a search for low-frequency radio variability in 944 bright ( > 4 Jy at 154 MHz ) unresolved , extragalactic radio sources monitored monthly for several years with the Murchison Widefield Array .
In the majority of sources we find very low levels of variability with typical modulation indices < 5 % .
We detect 15 candidate low frequency variables that show significant long term variability ( > 2.8 years ) with time-averaged modulation indices \overline { M } = 3.1 - 7.1 % .
With 7/15 of these variable sources having peaked spectral energy distributions , and only 5.7 % of the overall sample having peaked spectra , we find an increase in the prevalence of variability in this spectral class .
We conclude that the variability seen in this survey is most probably a consequence of refractive interstellar scintillation and that these objects must have the majority of their flux density contained within angular diameters less than 50 milli-arcsec ( which we support with multi-wavelength data ) .
At 154 MHz we demonstrate that interstellar scintillation time-scales become long ( \sim decades ) and have low modulation indices , whilst synchrotron driven variability can only produce dynamic changes on time-scales of hundreds of years , with flux density changes less than one milli-jansky ( without relativistic boosting ) .
From this work we infer that the low frequency extragalactic southern sky , as seen by SKA-Low , will be non-variable on time-scales shorter than one year .