Contents

CHAPTER 10: Quasi-Experimental Designs and Program Evaluation 315Key ConceptKey ConceptKey ConceptPrior to doing an experiment, we want to consider what major classes ofpossible explanations can be ruled out by our experimental procedure. Onlyby controlling all possible alternative explanations can we arrive at a definitecausal inference. In previous chapters, we referred to various uncontrolled factorsthat threaten the internal validity of an experiment as confounding factors(they are also called confounds). Several types of confounds were identified inearlier chapters (see especially Chapter 6). Campbell and Stanley (1966; Cook& Campbell, 1979; see also Shadish, Cook, & Campbell, 2002; West, 2010) haveidentified eight classes of confounds that they call threats to internal validity.You have already been introduced to some of these; others will be new. Afterreviewing these major threats to internal validity, we will be able to judge theextent to which various experimental procedures control for these kinds ofalternative explanations of a treatment effect.History The occurrence of an event other than the treatment can threaten internalvalidity if it produces changes in the research participants’ behavior. Atrue experiment requires that participants in the experimental group and in thecontrol group be treated the same (have the same history of experiences whilein the experiment) except for the treatment. In the laboratory, this is usuallyaccomplished by balancing or holding conditions constant. When doing experimentsin natural settings, however, the researcher may not be able to maintaina high degree of control, so confounding due to history can threaten internalvalidity. For example, suppose that you set out to test whether a college-levelcritical thinking course does, in fact, change students’ thinking. And supposefurther that you simply examined students’ performance on a critical thinkingtest at the beginning of the course and then again at the end of the course.Without an appropriate comparison group, history would be a threat to internalvalidity if events other than the treatment (i.e., the critical thinking course)occurred that might improve students’ critical thinking abilities. For instance,suppose many students in the course also accessed a website designed to teachcritical thinking that wasn’t required for the course. The students’ history, nowincluding the website experience, would confound the treatment and thereforepose a threat to the internal validity of the study.Maturation Participants in an experiment necessarily change as a function oftime. They grow older, become more experienced, and so forth. Change associatedwith the passage of time per se is called maturation. For example, supposea researcher is interested in evaluating children’s learning over a school yearusing a new teaching technique. Without a proper comparison, a researchermight attribute the changes in children’s performance between the beginningand the end of the school year to the effect of the teaching intervention when,in reality, the changes were simply due to a maturation threat to validity. Thatis, the children’s learning may have improved simply because their cognitiveabilities increased as they aged.Testing Taking a test generally has an effect on subsequent testing. Consider,for example, the fact that many students often improve from the initial test in a