Threats to the validity of emergency medical services evaluation: a case study of mobile intensive care units.

Much of the literature concerning emergency medical services evaluation has been criticized as unconvincing. Several sources of invalidity have comprised the interpretability of these studies. When true randomized experiments cannot be accomplished, quasi-experimental research designs offer greater interpretability than the more often used pre-experimental designs. In using quasi-experimental research designs, special attention must be given to threats to internal validity. A case study describes an evaluation of mobile intensive care units. The paper describes eighteen threats to the validity of the evaluation, as well as the methods used for their control. Whether or not evaluators can control all of the threats to the validity of their studies, these threats should be identified and their potential effects assessed wherever possible.