Low frequency variability and interstellar focusing

The authors present evidence which suggests that low frequency variability is a more common feature of extragalactic radio sources at low galactic latitudes. In the sample of radio sources monitored over a period of five years at Bologna, the mean galactic latitude of the variable sources is smaller than that of the non-variables to an extent that would occur with a chance of only 2% were the two groups drawn from the same parent distribution. (author)