The position in Britain regarding the collection of occupational data for locality studies is easily summarized. Almost any codification would be an improvement on the present situation. Many of the best known locality studies illustrate four common practices which I should regard as faults. The examples given below are purely illustrative. The list should not be taken either as exhaustive or as implying that the studies are bad ones; indeed they are chosen as examples largely because they are well known, and frequently they are well known because they are good studies, even if far from perfect.