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Sociology and Anthropology
SOAN 2120
David Walters

October 25, 2012 Experimental Designs - independent variable  dependent variable (differences in outcomes between two groups) - equivalent o average scores on the dependent variable and o scores on all other variables which are likely to affect the dependent variable - differences: manipulation - how do we establish the equivalence of two or more groups? o Random number tables o Objective: equal probability of being assigned to the group - Essential: (the groups are equivalent) - Conclusion: manipulation - Even with RA the two (or more) groups will not be exactly equal in all respects. - If the assignment is random, we can use tests of statistical significance (manipulation vs. random variation) - As n increases, individual differences decline - Without random assignment (RA): findings may be attributable to pre- existing differences between groups o What is this called? Selection bias - We could compare pre-test scores (dependent variable measures) before the experiment begins o Significantly different?  “equivalent”  proceed as if random assignment had taken place - What about all other potential confounding variables? - This happens automatically with random assignment Experimental Design Notation - Standard Notation (page 181) o R O1 X O2 o R O3 O4 - X represents an “experimental variable or effect” (the independent variable) - O represents a dependent variable observation. Subscripts (scripts) depict the observation; the exact ordering of the numbering is somewhat unimportant. - This experiment has a pre-test and post-test. - First (the top group) gets the change. - Second group is the controlled group. - The pre-test for the experiment group (O1) - The pre-test for the control group (O3) - Finally, the letter R denotes RA Example Experimental Designs (Many Groups) - Do “gimmick devices” improve drinking times? - Group 1: “drink through a bottle” - 3.9 seconds - Group 2: “regular bong” - 2.9 seconds - Group 3: “bottle bong” - 2.5 seconds - Different levels of the independent variable (different treatments) - This experiment would be diagramed as follows: - R X1 O1 - R X2 O2 - R O3 Notice that in this example, there is no pre-test Does this invalidate any of the findings? Internal Validity Whether the experimental condition (treatment) actually had an effect within the experiment itself. - History o When some event other then the experimental manipulation affects an outcome, or changes respondents o Before/after design o How can we know that the change that occurred as a result of the experimental manipulation, as opposed to all of the other things which respondents experiences between the pre-test and the post- test? - Maturation o Maturation is a subset of history o With the passage of time, people get tired, get bored, get hungry o Children subjected to a pre-test post-test design (before/after design)  Example:  Pretest: play with toys  Treatment: violent cartoon  Post-test: play with toys o Changes due to cartoon? Or maturation factors as the afternoon wears on? - Testing o Subjects are observed (tested) more than once o Early testing influences later testing o Example:  Does exposure to a cartoon will lead to more violent play- behaviour?  Children are viewed through a one-way mirror playing with toys (unobtrusive testing)  Dependent variable: observed number of ‘violent acts’  Even though the ‘testing’ procedure is unobtrusive, they may have become bored with their toys – ‘tested’ before o Measures of “performance” of any sort when measured more than
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