Institute:Office of National Coordinator (ONC) Workforce Training Curriculum
Component:Introduction to Health Care and Public Health in the U.S.
Unit:Evolution of and Trends in Health Care in the U.S.
Slide content:Hierarchy of Study Designs 9.1 Chart: The Hierarchy of Study Designs 9
Slide notes:It is not the intention of this lecture to study the designs themselves in great detail, but it is advantageous to have an idea of how the hierarchical study designs are organized. Continuing with the hierarchy ladder analogy, t here are rungs in that ladder, and if we knew what those rungs signified, it would be easier to understand what a study requires, in order for it to ascend or descend that hierarchy of evidence. At the bottom of the ladder, the lowest rungs signify expert opinions and physiologic studies. The major disadvantage of expert opinions is that they are opinions, and they are only as good as the experts who deliver them. One rung higher, in the hierarchical study design, are case studies. These are systematic observations without controls. What are controls? Studies that follow proper scientific methodology, and are designed to be true experiments, have two branches: the intervention group and the control group. The intervention group is comprise d of subjects where some intervention takes place and they are subjected to a change. The control groups are group s of subjects who do not undergo the intervention. Case studies are systematic, but they are observational studies, and they do not have true controls. One rung higher are case control studies. These are systematic studies. They do have controls, but they are defined by the outcome of interest. They are also known as retrospective studies because the outcome defines the analysis of data. One rung higher are cohort studies. These are true experiments with controls and intervention, and they are defined by exposure to the factor and the intervention and the control group. The two cohorts are followed prospectively over time and data is then analyzed. Then, at a higher level of the hierarchy of study designs are randomized controlled trials. In these studies there is an intervention group, and there is a control group, but both groups are randomized. Neither the subjects, nor the experimenters, know which group subjects are participating in. There is an equal probability of assignment of subjects to either the intervention group, or to the control group, and this makes the study more robust than cohort studies, case control studies, or other types of designs that are lower in the hierarchy of evidence. 9