Aresearcher assesses participants' attitude toward marijuana, presents them with a persuasive communication favoring liberalization of marijuana laws, and then reassesses their attitude toward the drug. however, between assessment 1 and assessment 2, the president also advocates liberalization of marijuana laws. if the researcher doesn't use a control group, any differences that he or she observes are likely to be caused by effects.

A researcher assesses participants' attitude toward marijuana, presents them with a persuasive communication favoring liberalization of marijuana laws, and then reassesses their attitude toward the drug. Though, among assessment 1 and assessment 2 the president also supporters liberalization of marijuana laws. If the researcher doesn't use a control group, any differences that he or she observes are likely to be caused by history effects. History effects talks about to events that occur in the environment that variation the conditions of a study distressing it outcome. Such a history event can happen before the start of an experiment or between the pre-test and post-test.