Understanding what determines workplace performance is important for a variety of reasons. In the first place, it can inform the debate about the UK's low productivity growth. It is also enables researchers to determine the efficacy of different organisational practices, policies and payment systems. In this paper we examine not the determinants of performance but how it is measured. Specifically, we assess the alternative measures of productivity and profitability that are available in the 2004 Workplace Employment Relations Survey (WERS).