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Experiments can be partitioned into two types: randomized experiments and quasiexperiments. Both types of experiments are used to estimate the effects of treatments and interventions. Estimating a treatment effect requires comparing what happened after a treatment was implemented with what happened after no treatment (or an alternative treatment) was implemented. In randomized experiments, the different treatments are assigned to participants at random, and in quasiexperiments, the treatments are assigned nonrandomly. Four prototypic quasiexperiments are described: before-after, interrupted time-series, nonequivalent group, and regression-discontinuity designs.

Before-After Comparisons

A before-after comparison is one of the simplest and most common, but also potentially most misleading, designs for estimating a treatment effect. In a before-after comparison, measurements collected before a treatment is introduced are compared to measurements collected after the treatment is ...

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