Internal and External Validity
Developing a research design should be more than just a matter of convenience (although practicality is an important element, which we touched on in the last section). Not all designs are created equally and there are trade-offs we make when opting for one type of design over another. The two major components of an assessment of a research design are its internal validity and its external validity. Internal validity basically means we can make a causal statement within the context of our study. We have internal validity if, for our study, we can say our independent variable caused our dependent variable. Often times, the major challenge is the issue of spuriousness. We have to ask if our design allows us to say our independent variable makes our dependent variable vary systematically as it changes and that those changes in the dependent variable are not due to some third or extraneous variable/factor.
The second basis for evaluating your research design is to assess its external validity. External validity means that we can generalize the results of our study. It asks whether our findings are applicable in other settings. Here we consider what population we are interested in generalizing to. We might be interested in adult Americans, but if we have studied a sample of first-year college students then we might not be able to generalize to our target population. As you can see here, the sampling method is the key. The quality of the sampling method you choose is directly tied to your ability to generalize the findings of one particular study to the entire population. Typically a representative sampling method gives us the best chance to generalize the findings to our target population, thus gives the study high external validity. By contrast, when a non-representative sampling methods is used, it reduces generalizability, i.e., the external validity of the study.