Efficient VLBI tests of relativistic models for superluminal motion require a complete sample of sources free from selection effects in orientation, coupled with an observational requirement for bright cores. Probably the best way to construct such a sample is to extract from a low frequency sample those sources with the highest radio luminosity, since these tend to have the strongest cores. One must then ask whether there are any sources included which would not be if they were differently oriented; that is, either the flux or the power falls below the limit for the sample when the core and jet flux is removed.