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

(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 that age group was down to 28.5% (Reference: The First Measured Century, T. Caplow, L. Hicks, and B. J. Wattenberg).

(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.