Objectives: The need for consistency and standardization of methods for economic appraisals has been recognized for some time and has led to the development of several sets of guidelines for economic evaluations and for costs. Despite this, considerable diversity is still apparent in applied studies. Some of these diversities might be defensible, and some might not. The objectives of this study are to explore sources of variations in the methods used in applied studies and to discuss the nature of these variations and the possibility of reducing some of them.

Methods: We first use a systematic approach to identify the major sources of variation in costing methods used in applied economic evaluations. We then compare the methods used with the recommendations made in available guidelines.

Results: Four possible sources of variation are identified. The first is where guidelines do not agree in their recommendations; therefore, it is not surprising that applied studies use different methods. The second is where guidelines agree in principle but provide little detail on how to comply with their recommendations; and the third is where a particular methodological issue is not discussed in guidelines. The fourth reason is simply lack of compliance with accepted guidelines.

Conclusions: Variability in costing methods used in applied studies raises questions about the validity of their results and makes it difficult to compare the results of different studies. We discuss the implications for the transferability and generalizability of results and suggest ways to minimize the variability in the methods so that the results of costing studies and economic evaluations can be of more value to policy-makers.