Data Collection Strategies
35 Internal Validity
In the preceding sections we reviewed three types of research: experimental, correlational and quasi-experimental. It is important to note that when it comes to internal validity, they are not considered equal. You will recall in the internal validity chapter, we briefly discussed internal validity. To recap, internal validity is the extent to which the study design supports the conclusion that changes to the independent variable were responsible for the observed changes in the dependent variable.
Of the three types of research, experimental, correlational and quasi-experimental, experimental research usually has the highest internal validity. This is because it addresses directional and third variable problems through manipulation and controlling for extraneous variables through random assignment (Crump et al., 2017). As Crump et al., (2017) state, if the average score on the dependent variable changes across conditions, it is likely that these changes are the result of the independent variable. On the other hand, correlational research is said to have the lowest internal validity. This is because if the average score on the dependent variable changes across conditions, it could be because of the independent variable. However, there could be other reasons, such as the direction of causality is reversed, or there is a third variable causing the differences in both the independent and dependent variables (Crump et al., 2017).
Quasi experimental research is considered in the middle of the two other types of research when it comes to internal validity. This is because the independent variable is manipulated in quasi experimental research; however, the lack of random assignment and experimental control can create other problems. Here is an example for better understanding.
Example
Suppose a researcher finds two similar firehalls i which to conduct a study on Post Traumatic Stress Disorder (PTSD). As part of her research, the researcher creates a PTSD awareness program and implements of the program at one of the two firehalls. At the end of the infield portion of her study she finds lower levels of PTSD in firefighters at the “treatment firehall,” than in the “control school (no program).” As she did not choose which firehall received the program based upon the number of firefighters with PTSD in each firehall, we can see that she did not have a directional problem with her study design. However, because she did not randomly assign firefighters to one of the two firehalls, it could mean that the firefighters at the treatment firehall differed somehow than the firefighter at the control firehall. In other words, it is the difference in the firefighters themselves (or something about their jobs, their superiors, etc.) that was responsible for the lower levels of PTSD at the one treatment firehall, not the PTSD awareness program that was applied.
Figure 6.1 demonstrates the differences in terms of internal validity as it relates to experimental, quasi-experimental, and correlational research. You will notice that there is some overlap in internal validity amongst the three types of research. For example, a poorly designed quasi-experiment can have a lower or equal level of internal validity than a well-designed correlational study. It is important to note that in addition to internal validity, it is important for the researcher to think about how to ensure a high level of other types of validity, such as external validity.
Media Attributions
- Internal Validity Scale
