The statement which represents an example of an observational study as required to be identified in the task content is; Choice D; Having customers fill out a questionnaire about their favorite brand of toothpaste.
Similar to fields which includes, but not limited to, social sciences and psychology, Statistically, an observational study as implied by the term is one which draws inferences from a sample to a population in which case, the independent variable is not under the control of the researcher which may be attributed to the avoidance of ethical concerns.
Hence, the answer choice D in which case, the independent variable is not under the control of the researcher represents such scenario.
Remarks:
A. Testing the effectiveness of a mouthwash by allowing one group to use it and comparing the results with those of a group that
doesn't use it
O B. Dividing a class into thirds, giving each third a different amount of time to read, and then testing comprehension
O C. Collecting the blood pressure readings of a group of elderly individuals in a small town
O D. Having customers fill out a questionnaire about their favorite brand of toothpaste.
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