A researcher must consider that a margin of error in a poll reflects only ______. A. data entry erroWhich is an example of an observational study? A. Researchers assign participants to teach either older or younger people how to use a computer to test whether participants appear more frustrated teaching older people. B. A manager uses sales records to compare the performance of different employees. C. A professor studies the relationship between democracy and per capita income using student graduation interview data. D. A political scientist uses data from a case study of 200 U.S. adults to examine the effect of media exposure on people's trust of government.rs B. errors from biased questions C. sampling error D. nonresponse bias