Researchers compare the body composition of cattle feeding on a grain diet with that of cattle feeding on a grass diet. This method of gathering data is classified as

a. a census.

b. a sample survey.

c. an experiment.

d. an observational study.

In an observational study, the individuals are observed and certain outcomes are measured. The variables are not under the control of researcher. While in an experimental study, the researcher controls the variables.

In the given case, the researcher is simply performing the observations. He is not controlling any variable or any conditions. He is gathering data from an already existing condition.

Therefore, this method of gathering data will be classified as an observational study.