Internal validity of any research design refers to the design's ability to make causal inferences from the data collected and the results of the study (Shadish, Cook, & Campbell, 2002). The research designs that offer the highest levels of internal validity are true experiments where the participants are randomly assigned to the conditions in the study. Quasi-experimental designs attempt to improve on the poor internal validity of correlational designs by allowing the researcher to control the assignment of participants to the treatment condition such as using some type of eligibility criteria, but due to the nature of the study the researcher cannot use random assignment. Most quasi-experimental designs are performed with intact groups that cannot be the subject of any random assignment (Shadish et al., 2002).
Metheny, Davis-Jackson, and Stewart (2010) performed a two-group quasi-experimental design to determine the effectiveness of an Aspiration Risk Reduction Protocol (ARRP) in critically ill, mechanically ventilated tube -- fed patients on an intensive...
Aside from the aforementioned problems with internal validity in quasi-experimental designs the Metheny et al. study has several threats to its internal validity:
1. Probably the most glaring threat to the internal validity of this study is that the researchers do not appear to have been blind to the conditions of the participants. The data was collected prospectively and data collection for the study was a permitted only on participants gave informed consent. It appears that the researchers knew which participants belong to which group. This could be a major threat to internal validity and to any generalizations of the study. A more useful way to collect data would be to not have the participant groups identified and to run the analysis without knowing which group was the treatment or control group.
2. Although there may be no way around this in some studies only getting data from a particular subset of the sample can threaten the internal validity of the study. In the current study, only those participants who gave…
