All experiments are about discovering the effects of causes. In this sense, humans always have been experimenters, from early man seeing whether striking a stone against another stone would start a fire, to the modern cook trying out new ingredients in a recipe to see how it changes the taste. All experiments have in common the deliberate manipulation of an assumed cause (striking a stone, adding a new ingredient), followed by observation of the effects that follow (fire, taste). This common thread holds for all modern scientific experiments, including the randomized experiments discussed in the previous chapter, and the quasi-experiments described in the present chapter.

This chapter focuses on critical thinking about causation in quasi-experiments. The reason for this focus on causation is not that other kinds of critical thinking are unimportant in quasi-experiments. To the contrary, every bit of critical thinking that was described in the previous chapter for randomized experiments also has to be done in quasi-experiments, such as choosing good independent and dependent variables, identifying useful populations of participants and settings to study, ensuring that the assumptions of statistical tests are met, and thinking about ways in which the results might generalize. However, the quasi-experimenter also has one more task to do – the critical thinking that takes the place of random assignment.

All tasks in life are easier when you have the proper tools. This chapter describes some of the basic tools we use for the task of critical thinking about causation in quasi-experiments.