The same instructor is used for all 3 classes, but a different text is used in each one.
All students take the same test at the end of the course.
A certain strain of corn is grown at 10 different locations. Four different fertilizers are used to determine which results in the largest yield of corn. Three different companies produce a particular type of electrical component. Random samples of components are selected from the output of each company. Components are put into service until they die.