This problem occurs when the groups become different after the experiment begins because more participants drop out of one of the groups than out of the other(s) for various reasons.

The United States' efforts to control global climate change are best described as __________

a. effective b. multilateral c. aggressive d. mixed

Which of the following is true of earmarks?

a. Though Congress banned earmarks in 2010, members are still able to secure funding for projects in their districts. b. They have become an increasingly popular way for members to win favor with their constituents. c. They are condemned by members of Congress as wasteful spending. d. They are the main reason that public approval of Congress is so low.