1. Diagram of a quasi-experimental design with two groups
Study Group
Before Intervention study group
after
Compare
Control group
Before control group

2. INTERVENTION STUDIES
The researcher manipulates a situation and measures the effects of the manipulation. Usually (but not always) two groups are compared, one in which the intervention takes place (e.g., treatment with a placebo).
Experimental studies
Quasi-experimental studies.

3. Experimental studies:
Experimental design is the only type of study design that can actually prove causation. In an experimental study, individuals are randomly allocated to at least two groups. One group is subjected to an intervention or experiment, while the other group (s) is not. The outcome of the intervention (effect of the intervention on the dependent variable/problem) is obtained by comparing the two groups.

4. The classical experimental study design has three characteristics:
Manipulation: the researcher does something to one group of subjects in the study.
Control: the researcher introduces one or more control group (s) to compare with the experimental group.
Randomization: the researcher takes care to randomly assign subjects to the control and experimental groups. (Each subject is given an equal chance of being assigned to either group, by assigning them numbers and “blindly” selecting the numbers for each group).

5. SELECTION OF STUDY DESIGN
In selecting the design of the study you have to consider the type of information you want to obtain and devise strategies to enable you to obtain that information. The selection of an appropriate study design depends on:
The state of knowledge about the problem
The nature of the problem and its environment.
The resources available for the research
The ingenuity and creativity
Objectives of the study.