February 2004 — Exclusive Series: SBR
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What the Words Mean
While experimental and quasi-experimental designs must include at least one comparison group, they may or may not include a control group. ... The hallmark, then, of experiments and quasi-experiments is that there be a group that can be used for comparison purposes, for example, between two or more experimental treatments, or between one or more experimental treatments and a control treatment.
The main difference between experiments and quasi-experiments is that in experiments, study participants are randomly selected from the population to which results of the study are to be generalized and/or randomly assigned to experimental and comparison groups, but not necessarily both. [In contrast,] quasi-experiments do not employ randomization. Random selection is not haphazard or arbitrary. It follows a specified procedure using a table of random numbers or a computer program for random selection.
Random Assignment
Random assignment refers to the use of a random numbers table or computer program to assign members of the sample, whether randomly selected or not, to experimental or comparison groups. The process of random assignment helps ensure that any differences between the experimental and comparison groups, before the start of the experiment, are due to chance alone (i.e., due to random sampling fluctuations).
When both random selection and random assignment are not feasible, researchers must determine which is more important to the claims they want to be able to make on the basis of the results. The U.S. Department of Education's Institute of Education Sciences, which oversees the What Works Clearinghouse, currently places emphasis on random assignment, the rationale being that random selection is seldom possible in the real world of education practice.
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Reference
Committee on Scientific Principles for Education Research. 2002. Scientific Research in Education. National Academy Press: Washington, D.C.
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