Part 1 of this tutorial presents each of the major threats to internal validity, with definitions and examples of each as applied to a hypothetical experiment. Part 2 consists of a set of 36 problems; users are asked to indicate whether a hypothetical experiment is internally valid or not, and if not valid, to indicate the threat to internal validity.
This site contains a program that assists users in drawing a random sample and randomly assigning subjects. Lesson 2 in the Tutorial explains the difference between random sampling and random assignment, and then describes how to randomly assign 40 subjects to four conditions; Lesson 3 shows how to randomly assign 40 subjects in blocks of four.
