| Summary: | Abstract Background Sustainability is, at least in principle, an important criterion for evaluating global health and development programs. The absence of shared metrics for success or achievements in sustainability is however critically lacking. We propose a simple metric, free of causal inference, which can be used to test different empirical models for the sustainment of health outcomes. Methods We follow the suggestion of Chambers and use “sustainment” to refer to the verifiable and measured extent to which a health indicator has evolved over time. The sustainment index of a health indicator (Y) advanced by a program is based on a simple-to-calculate approximation of the derivative of Y over time (T0: baseline, T1: endline, and T2: post-project), based on the ratio of the slope of YT1-T2 over YT0-T1. SI(Y) = 1+ (YT1-T2 / YT0-T1). Results This construct provides three clear benchmarks: SI = 0, when the health indicator returns to baseline value post-project (YT2 = YT0); SI = 1, when the endline-post-project trend is a plateau; and SI = 2, when the progress slope during program is uninterrupted post-program. We find strong correlation (r2 = 0.922) between the SI and independent practitioners’ rating of indicator trends. The SI shows different levels of achieved sustainment for a range of indicators in a published ex-post sustainability study. And we find that the SI can be computed for large national datasets for two types of indicators. Conclusions The Sustainment Index has limitations and conditions of applicability, but it can be applied to different datasets and studies to provide a reliable dependent measure of the level of sustainment of health outcomes from one period of time to the next. The Index will need additional testing, and future evaluation-research work will need to consider index performance under different situations. The Sustainment Index has the potential to provide a standard metric to build evidence through more systematic research on sustainment and sustainability.
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