Levels of evidence: Description of level 2b

2b: Evidence obtained from at least one other type of well designed quasi-experimental study

Quasi-experimental study: A non-randomized pre-post intervention study.

A quasi-experimental study is usually performed due to:

ethical considerations

inability to randomize patients/ locations

the need to intervene quickly.

Although quite popular, they lack the internal validity of true experimental studies- RCTs. This is largely due to the inability to adjust for known and unknown confounders by employing randomization.

There are many types of quasi-experimental studies. Details of such studies may be found in the following article:


A different take on such studies may be found here:


These study designs are rated above observational study designs because they are ‘experimental’ in nature- they allow testing of a hypothesis. However, causality cannot be directly inferred from such studies (unlike in true experimental study designs).

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