A quasi-experiment is an empirical intervention study used to estimate the causal impact of an intervention on a population without random assignment. Quasi-experimental design allows the researchers to control the assignment to the treatment condition, but using some criteria different from random assignment.

In true experiments, treatment and control group are identical in every way except for the treatment variable. In quasi-experiment, this may not be true.

Quasi-experimental design is a useful tool in situations where true experiments cannot be used for ethical or practical reasons.

Quasi-experiments use different statistical methods to control for confounding variables; however, because partecipants are not randomly assigned, there is always a risk of selection bias and other threats to internal validity.

Types of quasi-experiments

Nonequivalent groups design

The researchers choose groups that appear similar, but only one of the groups has experienced treatment. This is the most common type of quasi-experiment.

Regression discontinuity

Many potential treatments that researchers wish to study are designed around an essentially arbitrary cutoff, where those above the threshold receive the treatment and those below it do not.

Near this threshold, the differences between the two groups are often so minimal as to be nearly nonexistent. Therefore, researchers can use individuals just below the threshold as a control group and those just above as a treatment group.

E.g.: Some high schools in the United States are set aside for high-achieving students, who must exceed a certain score on a test to be allowed to attend. Those who pass this test most likely differ systematically from those who do not. However, since the exact cutoff score is arbitrary, the students near the threshold—those who just barely pass the exam and those who fail by a very small margin—tend to be very similar, with the small differences in their scores mostly due to random chance. You can therefore conclude that any outcome differences must come from the school they attended.

To test the impact of attending a selective school, you can study the long-term outcomes of these two groups of students (those who barely passed and those who barely failed).

Natural experiments

An external event or situation (“nature”) results in the random or random-like assignment of subjects to the treatment group.

Even though some use random assignments, natural experiments are not considered to be true experiments because they are observational in nature.

Although the researchers have no control over the independent variable, they can exploit this event after the fact to study the effect of the treatment.

References

• https://en.wikipedia.org/wiki/Quasi-experiment
• https://www.scribbr.com/methodology/quasi-experimental-design/