Which of the following is a before-after experiment?

Select one:

a. An experiment in which the marketer observes the after-effect of natural occurrences on customer behavior.

b. An experiment in which the marketer observes sales before an experimental manipulation and then again after, in a single test group.

c. An experiment with two groups, in which the marketer compares the change in sales for one group that got the same old marketing in time 1 and time 2, versus for another group that got the old marketing in time 1 and new marketing in time 2.

d. An experiment with two groups, in which the marketer compares sales for one group that got the old marketing versus another group that got the new marketing.