Quasi-experimental study designs series-paper 4: uses and value.

Quasi-experimental studies are increasingly used to establish causal relationships in epidemiology and health systems research. Quasi-experimental studies offer important opportunities to increase and improve evidence on causal effects: (i) they can generate causal evidence when randomized controlled trials are impossible; (ii) they typically generate causal evidence with a high degree of external validity; (iii) they avoid the threats to internal validity that arise when participants in non-blinded experiments change their behavior in response to the experimental assignment to either intervention or control arm (such as compensatory rivalry or resentful demoralization); (iv) they are often well-suited to generate causal evidence on long-term health outcomes of an intervention, as well as non-health outcomes such as economic and social consequences; and (v) they can often generate evidence faster and at lower cost than experiments and other intervention studies.