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Experimental Designs Essay

1286 words - 6 pages

Experimental Designs
Control Group Pre-test/Post-test Design
Threats to Internal Validity
Threats to External Validity
Post-Test only Control Group Design

To establish whether two variables are causally related, that is, whether a change in the independent variable X results in a change in the dependent variable Y, you must establish:
1) time order--The cause must have occurred before the effect;
2) co-variation (statistical association)-- Changes in the value of the independent variable must be accompanied by changes in the value of the dependent variable;
3) rationale-- There must be a logical and compelling ...view middle of the document...

Researchers may substitute matching for random assignment. Subjects in the two groups are matched on a list of characteristics that might affect the outcome of the research (e.g., sex, race, income). This may be cheaper but matching on more than 3 or 4 characteristics is very difficult. And if the researcher does not know which characteristics to match on, this compromises internal validity.
2) Controls the timing of the independent variable (treatment) and which group is exposed to it.
Both group experience the same conditions, with the exception of the experimental group, which receives the influence of the independent variable (treatment) in addition to the shared conditions of the two groups.
3) Controls all other conditions under which the experiment takes place.
Nothing but the intervention of the independent (treatment) variable is assumed to produce the observed changes in the values of the dependent variable.
The steps in the classic controlled experiment are:
1) randomly assign subjects to treatment or control groups;
2) administer the pre-test to all subjects in both groups;
3) ensure that both groups experience the same conditions except that in addition the experimental group experiences the treatment;
4) administer the post-test to all subjects in both groups;
5) assess the amount of change on the value of the dependent variable from the pre-test to the post-test for each group separately.
These steps are diagramed as follows:

R O1 X O2
R O1 O2

This diagram can be expanded upon as in the following table:
Scientific Random Assignment of Subjects to: 1st observation (measurement) of the dependent variable
O1 = Pre-test Exposure to the Treatment (X) (independent variable) 2nd observation (measurement) of the dependent variable
O2 = Post-test
Experimental Group Experimental Group's average score on the dependent variable X Experimental Group's average score on the dependent variable
Control Group Control Group's average score on the dependent variable Control Group's average score on the dependent variable

The difference in the control group's score from the pre-test to the post-test indicates the change in the value of the dependent variable that could be expected to occur without exposure to the treatment (independent) variable X.

Control group - control group = control group difference
pre-test score post-test score on the dependent variable

The difference in the experimental group's score from the pre-test to the post-test indicates the change in the value of the dependent variable that could be expected to occur with exposure to the treatment (independent) variable X.

Experimental group - experimental group = experimental group difference
pre-test score post-test score on the dependent variable

The difference between the change in the...

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