Where there is an element of human judgement in the selection of people for the sample, and every person in the population does not have an equal chance of being selected. The quality of the sample is largely dependent on the judgement of those who select people for the sample. Unlike random sampling, with non-random sampling, sampling error cannot be calculated. Non-random sampling techniques are, however, generally more convenient and less costly to carry out. Non-random sampling methods consist of quota, convenience and snowball sampling.