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).

Answer:a. 4)b. 4)Step-by-step explanation:Hello!An observational study is one where the investigator has no control or intervenes on it. He just defines the variable of interest and merely collects and documents the information.An experimental study or experiment is one where the investigator intervenes by defining the variable of interest and artificially manipulates the study factor. It also one of its characteristics the randomization of cases or subjects in groups (two or more, depending on what is the hypothesis of study).(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 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 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 observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured. In this item, the researcher merely obtained the records of the population that were 19 or younger over several years and compared the obtained percentages. This is a clear example of an observational study, the researcher did not manipulate any factor or variable, he just looked up the information and documented it.(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 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 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 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 treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.In this item, the students were randomly assigned to one of two groups and each group was given a test, group one: multiple choice and group two: essay. This is an example of an experimental study, after the class, the researcher controlled the variable "type of test" giving one type to each group and the subjects were randomly selected fro the groups assuring that they will not receive biased results. And at the end of the experiment, the response variables "test scores" where compared.I hope it helps!