A researcher is interested in the relationship between depression and the loss of a loved one. The individuals of interest are bereaved adults and nonbereaved adults. The variables in this study receive no manipulation, and depression is measured in both sets of adults. This is an example of .

A group comparison study

Explanation:

In group comparison study, an experiment compares two groups(the beareaved and nonbeareaved adults in the above study) such as in testing the effects of a treatment where a "comparison group" receives either an alternative treatment or no treatment so that it functions as a source of "counterfactual" causal inference for the purpose of the experiment.

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this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.