experimental, quasi-experimental and descriptive research

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1) Experimental Research is that which tests causal relationships by observing the behaviour of the subject under conditions where some variables are controlled and others manipulated.

Strictly speaking, the term experiment should be confined to those actions or series of actions where it is possible to do all of the following:

Randomly assign the subjects of the experiment to either an experimental group (to which something is done) or a control group (to which the thing done to the experimental group is not done).
Manipulate (do something to) the experimental group.
Ensure that in all other important aspects, the factors affecting the experimental and control groups remains the same.

Experimental research is regarded by many as the optimum quantitativemethodology for obtaining reliable information about treatment or inter-vention effect (McMahon, 1994; Mulhall, 1994; Sibbald and Roland,1998; Donnan, 2000; Richardson, 2000; Polit et al., 2001).

Correlational research:

In correlational research, the investigator deliberately seeks to examinelinks (or relationships) between variables without introducing an inter-vention. The purpose is often to generate hypotheses that can ...