Let’s say a consumer goods company wishes to conduct a survey of its clients. They choose four goods at random from the ten goods groups and ask consumers in the chosen goods groups to score their experience on a scale of one to ten. Which of the following sampling techniques is being used in this situation?

A study of the salaries of professors at a University shows that the average salary for female professors is considerably less than the average male salary. Further investigation broken down by department (English, Physics, etc.) shows that the average salary for female professors is more than the average male salary. This apparent contradiction is an example of