When subjects are not found at home in a social survey, the question arises whether the subsample encountered at home, on the first or subsequent visits, is random or biased. A procedure is presented by which this question can be statistically tested, by comparing the decline rate in unfound subjects, over repeated visits, with those expected if the subsample were random or strongly biased. The randomness of the subsamples can be compared between the first and subsequent visits. The procedure can be carried out during a programme of revisits, to check quickly whether a satisfactory sample is being obtained.