Trochim (2006) states that a quasi-experimental design is one that looks a bit like an experimental design but lacks the key ingredient -- random assignment. He notes that his mentor [Don Campbell] used to refer to quasi-experiments as "queasy" (2006) experiments because they give the experimental purists a queasy feeling.
With respect to internal validity, they often appear to be inferior to randomized experiments. But there is something compelling about these designs; taken as a group, they are easily more frequently implemented than their randomized cousins (Trochim 2006).
The most important part of both experimental and quasi-experimental research is the measure of the dependent variable, which it allows for comparison. Some types of data are very straightforward, but there are other measures, but there are other types of data that are completely subjective. In cases where the data is highly subjective, the quasi-experiment...
For example, efficacy and accuracy are a debatable subject when it comes to experimental research because many believe that real-life environments are difficult to be reduced to a small set of treatment and outcome variables because they are simply much too complex. There are also some ethical concerns that have become controversial; many treatments are applied without subjects' knowledge. Quasi-experimental research design, on the other hand, is often used in settings where people or groups cannot be randomly assigned for ethical reasons.
Reference:
Trochim, William K. & Donnelly, James P. (2006). The research methods knowledge base.…
