What are the three types of experimental designs (e.g. pre-test-post-test)? What would be the strengths and weaknesses of each?
Interesting questions! Let's take a closer look.
1. What are the three types of experimental designs (e.g. pre-test-post-test)?
True Experimental Design has greater control and refinement, greater control of validity. A randomized experiment is a type of experimental design in which participants are randomly assigned to a treatment or control group (http://www.tulane.edu/~panda3/ME/mechapter2/mechap2pg3.htm), such as the three types of experimental designs discussed below:
(1) Pretest-posttest control group
With this randomized experimental design, the aim is to study the effect of an influence on a carefully controlled sample. This design has been called "the old workhorse of traditional experimentation." If effectively carried out, this design controls for eight threats of internal validity. Data are analyzed by analysis of covariance on posttest scores with the pretest the covariate (http://www.okstate.edu/ag/agedcm4h/academic/aged5980a/5980/newpage2.htm).
The total population of participants is randomly divided into two samples; the control sample, and the experimental sample. Only the experimental sample is exposed to the manipulated variable. The researcher compares the pretest results with the posttest results for both samples. Any divergence between the two samples is assumed to be a result of the experiment (http://en.wikipedia.org/wiki/Experimental_techniques).
Let's say, for the sake of argument, that you do not trust randomization to give you an experimental and control group that are initially equivalent (at least on average) in post-prandial blood sugar measured in the office. (I'm referring to the hypothetical situation I set up in the Posttest-Only Control Group Design discussion.) This would actually put you in good company, which is one reason why pretest-posttest approaches have been so popular. Anyway, you decide that you want to obtain initial measurements before you start your experiment. After you ...
By discussion and example. this solution exmamines the three types of experimental designs (e.g. pre-test-post-test), including the strengths and weaknesses of each.