Which technique for gathering data (observational study or experiment) do you think was used in the following studies? Explain your answer.

(a) The U.S. Census Bureau tracks population age. In 1900, the percentage of the population that was 19 years old or younger was 44.4%. In 1930, the percentage was 38.8%; in 1970, the percentage was 37.9%; and in 2000, the percentage in the age group was down to 28.5% (The First Measured Century,T. Caplow, L. Hicks, B. J. Wattenberg).

This is an experiment because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.

This is an observational study because a treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.

This is an observational study because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.

This is an experiment because a treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.

(b) After receiving the same lessons, a class of 100 students was randomly divided into two groups of 50 each. One group was given a multiple-choice exam covering the material in the lessons. The other group was given an essay exam. The average test scores for the two groups were then compared.

This is an observational study because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.

This is an experiment because a treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.

This is an experiment because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.

This is an observational study because a treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.