The categorisation of occupation in identified skeletal collections: a source of bias?

Identified skeletal collections, that is, skeletons for which sex, age at death and occupation at death are known, have been used to test methods for recording entheseal changes (EC). By testing methods on identified collections, the sensitivity of EC for recording activity levels can be ascertained prior to applying the methods to test hypotheses in archaeological contexts. However, the definition of occupational categories used for this research may, in itself, be a source of bias. The aim in this study was to test how categorising occupation affected the interpretation of EC. Male skeletons (n = 211) from two Portuguese identified skeletal collections were used. Three methods for categorising occupations, all of which have been previously published, were used each dividing occupations into five, three and two categories, respectively. Fibrocartilaginous entheses were recorded and EC scored as present/absent. Results showed that the method for categorising occupation affected the frequencies of EC found in occupational categories for specific entheses. Frequencies that were significantly different between occupational categories for one method were not necessarily significant for others. This demonstrates that the sensitivity of the occupational categorisation does affect the results. However, using logistic regression, we found age to have a greater effect than occupation. These results demonstrate the need for standardised occupational categories, as well as the importance of considering age.