Question: Identify which of these types of sampling is used in each of the following situations: random sample, systematic sample, convenience sample, stratified sample, or cluster sample.

a. To find the average number of people in a household in a certain state, randomly pick six counties in that state, and then ask each and every household in those six counties how many people reside there.

b. To find out if students in public schools will eat frozen yogurt instead of ice cream, first group the young people according to grade level: elementary school, middle school, and high school. Then take a random sample of fifteen from each group and have them do a blind taste test to see if they like the frozen yogurt enough to eat it in place of ice cream.

c. To decide on which magazine to advertise in, the subscribers are assigned numbers. Then, using a computer random number generator, 90 subscribers are randomly selected to be surveyed as to their buying habits.

d. To conduct a study on how patients feel after they leave a certain Hospital, every 20^th patient is given a survey before they are discharged.