To investigate a possible association between chocolate consumption and depression, a researcher had 20 volunteers who suffer from depression and 20 volunteers who do not suffer from depression keep food journals for one month. He then used the information in the journals to determine the amount of chocolate consumed for each person. The resulting data were then used to compare chocolate consumption for the two groups. This is an example of

(A) A census
(B) A random sample
(C) An experiment
(D) An observational study



S-IC.3: Recognize the purposes of and differences among sample surveys, experiments, and observational studies; explain how randomization relates to each.

Correct answer and commentary

The correct answer to this item is Option (D). About 59% of students answered this item correctly. The researcher uses two groups of volunteers to determine whether or not there is an association between chocolate consumption and depression. This is an example of an observational study because the researcher cannot randomly assign participants to different levels of depression and the researcher did not randomly assign participants to eat or not eat chocolate. The results of this study could make a claim about an association between depression and chocolate consumption (if the data support this) but not about the reason for this association because it is possible some other factor (e.g., genetics) affects both depression and chocolate consumption. Such results may influence future research studies which seek a causal explanation using a different design.

This is not a census the study did not include all people from an identified population. The context does not describe a random sample because volunteers were used. The most popular distractor was Option (C) with a response rate of about 33%. In order for a study to be an experiment, the researcher would have to take the 40 people and randomly assign them to a treatment group and a control group. For example, one group could consume chocolate and the other group not consume chocolate. Then, the researcher could record the level of depression for each person after a period of time and compare the difference in depression levels between the two groups to make a stronger claim regarding the relationship between chocolate consumption and depression. Since the researcher did not use this kind of control, this is not an example of an experiment.

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