Abstract
Despite its frequent use by sociologists there has been relatively little discussion of the practical difficulties involved in operationalizing the Office of Population Censuses and Surveys (OPCS) system of social class allocation. Using social class coding data from a national study of child development this paper examines one problem which occurs when using the OPCS scheme - illegal combinations of employment status and occupational codes. Attention is only focused on occupations falling into Social Class I and for these professional occupations it is shown that illegal combinations are the result of the OPCS conceptualization of the professions.