ABSTRACT
This chapter begins with a discussion of causation and causal hypothesis testing and then introduces the concepts of mediation and moderation of causal relations. Testing causal hypotheses in an experiment involves manipulating an independent variable (experimental treatment or intervention) and assessing effects on a dependent variable. Experiments can be conducted in laboratory or field settings for purposes of testing basic theory or achieving an applied goal. A good experiment will have high internal validity. That means if the independent variable results in changes in the dependent variable, we can conclude that there is a causal relation between them, if all irrelevant factors have been ruled out. Potential threats to internal validity are listed and discussed. Many of these threats can be ruled out if participants in the experiment are randomly assigned to experimental treatments. Random assignment is the essence of “true” experiments. The chapter closes with an introduction to external validity, the extent to which findings from a research study can be generalized to other populations and settings.
A wide range of potential research designs will be covered in the chapters that follow. They can be categorized roughly into either experimental or nonexperimental strategies. Experimental methods involve research in which participant actions are limited or in some way constrained by the controlled manipulation of variables determined by the researcher. Ideally, experiments should be characterized by random assignment, a procedure used to place participants, by chance alone, into the different conditions, which enables strong inferences of causality. Quasi-experimental methods are a variant of experimental methods lacking in random assignment, but as in experiments, participants are exposed to some form of variable manipulation imposed by the researcher. For example, we might be interested in the effects of National Merit Scholarships on college performance. Obviously, a researcher cannot randomly assign high school seniors to receive or not receive a Merit scholarship, because of ethical concerns and the fact that receipt of the award is determined by scores on a test. Quasi-experimental methods (Chapter 9) are an attempt to make inferences about possible variables responsible for the outcome differences between recipients and non-recipients of the scholarship, and to disentangle the effects of the scholarship from other variables such as intelligence or work ethic.
