Myths and Misconceptions of Within-Cycle Correction: A Guide for Modelers and Decision Makers

Commonly used decision-analytic models for cost-effectiveness analysis simulate time in discrete steps. Use of discrete-time steps can introduce errors when calculating cumulative outcomes such as costs and quality-adjusted life-years. There are a number of myths or misconceptions concerning how to correct these errors and the need to do so. This tutorial shows that, by neglecting to apply within-cycle (sometimes referred to as half-cycle or continuity) correction methods to the results of discrete-time models, the analyst may arrive at the wrong recommendation regarding the use of a technology. We show that the standard half-cycle correction method results in the same cumulative outcome as the trapezoidal rule and life-table method. However, the trapezoidal rule has the added advantage of applying the correction at each cycle, not just the initial and final cycle. We further show that the Simpson's 1/3 rule is more accurate than the trapezoidal rule. We recommend using the Simpson's 1/3 rule in the base-case analysis and, if needed, showing the results with other methods in the sensitivity analysis. We also demonstrate that both the trapezoidal and Simpson's rules can easily be implemented in commonly used software.