I assess your attitude toward marijuana,present you with a persuasive communication favouring liberalization of marijuana laws,and then reassess your attitude toward the drug.However,between assessment 1 and assessment 2,the President also advocates liberalization of marijuana laws.If I don't use a control group,any differences I observe are suspect on the grounds of

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The President's advocacy for liberalization of marijuana laws is an external event that could influence attitudes towards marijuana, regardless of the persuasive communication used. This is an example of a history effect, which can impact both the treatment and control groups, making it important to have a control group to compare any observed changes in attitudes. Maturation refers to natural changes in participants over time, while testing refers to the influence of previous assessments on the results. Regression toward the mean refers to the statistical phenomenon where extreme scores on a measure tend to become less extreme over time due to chance and measurement error, but this is not relevant in this scenario.

I Assess Your Attitude Toward Marijuana,present You with a Persuasive