History can pose a threat to internal validity when the group under study experiences an event--unrelated to the treatment--which has an impact on their performance on the posttest. For example, if a researcher is evaluating a drug prevention program, and during the time of the study a popular celebrity dies from a drug overdose, the effect (or lack of effect) that is noticed may have been caused by the group's reaction to the celebrity's death rather than the program that was implemented. 

How do you think a comparison group can alleviate this threat to internal validity? 

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Copyright © 1997 Wendy Martin. All rights reserved.