In an experiment, subjects are given a pretest in an effort to assess their attitudes about a new federal sentencing law that Congress is debating. Subjects assigned to the experimental group are given a series of lectures by experts on the impact the new law will have on prison populations. Immediately after the last lecture, a posttest is given to measure any attitude changes. Anticipating what the test is designed to measure (because of the lectures), those in the experimental group are subject to the threat of:.