A quasi-experimental study is a research design that shares similarities with experimental research but lacks randomization and/or a control group. In other words, it involves manipulating the independent variable but lacks some of the key elements of a true experimental design, such as randomly assigning participants to different groups or conditions.
Quasi-experimental studies are often used when it is not feasible or ethical to conduct a randomized controlled trial, such as when the intervention is already in place, or when it is not possible to withhold treatment from a control group. This type of study design can be useful in evaluating the effectiveness of interventions in real-world settings, but it is important to note that they are typically considered to be less rigorous than randomized controlled trials and may be subject to more sources of bias.