Costing and Cost Analysis in Randomized Controlled Trials: Caveat Emptor
This article provides an overview of the central issues regarding cost valuation and analysis for a decision maker's evaluation of costing performed within randomized controlled trials (RCTs). Costing involves specific choices for valuation and analysis that involve trade-offs. Understanding these choices and their implications is necessary for proper evaluation of how costs are valued and analyzed within an RCT and cannot be assessed through a checklist of adherence to general principles. Resource costing, the most common method of costing, involves measuring medical service use in study case report forms and translating this use into a cost by multiplying the number of units of each medical service by price weights for those services. A choice must be made as to how detailed the measurement of resources will be. Micro-costing improves the specificity of the cost estimate, but it is often impractical to precisely measure resources at this level and the price weights for these micro-units may not be available. Gross-costing may be more practical, and price weights are often easier to find and are more reliable, but important resource differences between treatment groups may be lost in the bundling of resources. Price weights can either be nationally determined or centre specific, but the appropriate price weight will depend on perspective, convenience, completeness and accuracy. Identifying the resource types and the appropriate price weights for these resources are the essential elements to an accurate valuation of costs. Once medical services are valued, the resulting individual patient cost estimates must be analysed. The difference in the mean cost between treatment groups is the important summary statistic for cost-effectiveness analysis from both the budgetary and the social perspectives. The statistical challenges with cost data typically stem from its skewed distribution and the resulting possibility that the sample mean may be inefficient and possibly inappropriate for statistical inference. Multivariable analysis of cost is useful, even if the data come from an RCT, but the same distributional problems that affect univariate tests of cost also affect use of cost as a dependent variable in a multivariable regression analysis. The generalized linear model (GLM) overcomes many of the problems of more common cost models, but caution must be used when applying this model because it is prone to mis-specification and precision losses in data with a heavy-tailed log error term. Attention to the appropriate cost valuation and analysis techniques reviewed here will help bring the same level of rigor and attention to the methodological issues in cost valuation as is currently applied to clinical evidence within RCTs.
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