Biases, selection effects and image multiplicities in the Jodrell Bank-VLA gravitational lens survey

In this paper we discuss the biases and selection effects relevant to the Jodrell Bank-VLA gravitational lens survey (JVAS). We present a method for the determination of the magnification bias and apply this to the luminosity function of the flat-spectrum radio source population. The magnification bias in our survey is lower than in optical surveys, as expected qualitatively from the steeper optical luminosity function slopes. The theoretical predictions for the number of sources of each multiplicity, which are based on lenses modelled as isothermal potentials with external shear, are compared with the findings of the survey; five-image events are detected more frequently than expected.