Practical Data Analysis with JMP®

Experimental and Observational Studies

In Chapter 2 we discussed the difference between experimental and observational studies. In an experiment, the investigator creates a controlled setting in which she or he can deliberately manipulate the values of some variables (factors or independent variables) and measure the values of one or more response (or dependent) variables. We can confidently draw inferences about causation based on well-designed experiments. In contrast, researchers exercise little or no control in observational studies. They gather data, sometimes after the fact, about events or subjects and then use methods that we've studied to analyze the evidence of interdependence among variables. Even the best-designed observational study ...