Spillover effects from healthcare interventions: A scoping review and framework for identification and measurement

Abstract

Background:
There is an increasing awareness among researchers and policymakers of the potential for healthcare interventions to have consequences beyond those initially intended. These ‘spillover effects’ result from the complex features of healthcare organization and delivery, and can either increase or decrease overall effectiveness. Their potential influence has important consequences for the design and evaluation of interventions, and for decision-making. However, consideration of spillovers remains partial and unsystematic. We develop a comprehensive framework for the identification and measurement of spillover effects resulting from changes to the way in which healthcare services are organised and delivered.

Methods:
We conducted a scoping review to map the existing literature on spillover effects in health and healthcare interventions, and used the findings of this review to develop a comprehensive framework to identify and measure spillover effects.

Results:
The scoping review identified a wide range of different spillover effects, either experienced by agents not intentionally targeted by an intervention or representing unintended effects for targeted agents. The existence of spillover effects can violate crucial assumptions required for the validity of non-experimental comparative evaluation designs, but these necessary assumptions were rarely considered. Unanticipated behavioural responses, complementarities and resource sharing in the production of healthcare were frequently cited as mechanisms driving the detected spillover effects. We propose a taxonomy of spillover effects, classified based upon the outcome and the unit experiencing the effect: within-unit, between-unit, and diagonal spillover effects. We then present the INTENTS framework; Intended Non-intended TargEted Non-Targeted Spillovers. The INTENTS framework considers the units and outcomes which may be affected by an intervention, and the mechanisms by which spillover effects are generated.

Conclusions:
The INTENTS framework provides a structured guide for researchers and policymakers when considering the potential effects that healthcare interventions may generate, and the steps to take when designing and evaluating such interventions. Application of the INTENTS framework will enable spillover effects to be appropriately addressed in future evaluations and decision-making, ensuring that the full range of costs and benefits of interventions are correctly identified.

Keywords – Health policy; Spillover effects; Evaluation; Healthcare Economics and Organizations; Program Evaluation; Unintended effects

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