Academic Research for MDs – Choosing a Study Design
In my last post I laid out how to construct a research question. Once you have a clear study question the next step is figuring out how you are going to answer that question. For this you need to determine the appropriate study design for your project. In this post I’ll go through several categories of study design and examples of appropriate designs for certain types of studies.
There are two basic categories of study design, observational and interventional. Observational studies are exactly what you would suspect. The investigators observe what is naturally occurring in a system. The investigator tries to interfere with the system as little as possible.
There are several types of observational studies. Cohort studies are a types of observational study that follows a group over time either prospectively or retrospectively. They often describe the incidence or natural history of a condition, commonly a chronic disease. They analyze predictors or risk factors for various outcomes. Retrospective cohort studies are good for investigating causes of conditions and defining their incidence. They are generally much less costly in terms of time and money. Investigators are limited in retrospective studies by the quality of variables they can collect. You can only analyze the data that has been collected. If it wasn’t collected, you can’t go back and get it so if you’re looking a variable that is rarely collected you may want to do a prospective study. Prospective Cohort studies are also good for defining incidence and risk factors. Investigators also have the advantage of collecting variable exactly how they want them to be collected. Unfortunately, prospective cohort studies are a very inefficient method of studying rare outcomes. Ten years of prospective data requires ten years to collect. Ten years of retrospective data can be collected in a fraction of the time, saving time and money.
A second type of observational study is the cross-sectional study. In a cross-sectional study the investigator makes all measurements at a single moment in time for each subject. This is good to investigate prevalence of a variable at a point in time but you obviously can’t track change in variables over time.
A third type of observational study is the case-control study design. This design classically has two groups based on outcome. Group 1 has outcome A while group 2 has outcome B. This design gives a high yield of information from relatively few subjects. This model can be an efficient design for investigation of diseases with long latent periods, but can’t measure incidence or prevalence.
The second category of study design is the interventional study. These studies insert the investigator into the system being analyzed. The investigator generally intervenes in one group of subjects while not intervening in the other. Interventional studies are subcategorized by method of segregating the groups. Subjects can be segregated randomly, generally by a computer program, or non-randomly as in subject self-selection when a subject volunteers for the intervention group. Randomized trails are preferred because randomization controls for confounding variables that the investigator is unaware of or is not collecting.
Interventional studies are also classified by who knows which subjects receive the intervention in question. If neither the subject nor the investigator measuring the variable are aware of the subjects grouping then the study is double blinded. If the subject is unaware of their grouping but the investigator measuring the variable is aware of the subjects group or vice versa, the study is single blinded. Pharmacists, technicians, physicians etc. can all be blinded to prevent bias from being introduced into study results. The “gold standard” of interventional studies is the randomized double blind trial, but these tend to be very costly and complicated.
In the end the best study design is the one that answers your study question, because performing a study only to be unable to answer your research question is a waste of both your time and money which are scarce resources for most researchers.
In the next post I’ll outline the creation of your study protocol. As always if you need assistance or advice on any part of the research process in the mean time, don’t hesitate to contact us here at the center. My contact information is below. We’d be glad to help.
Daniel Curran MD
Clinical Research Fellow
Clinical and Translational Research Support Center
University of Louisville Division of Infectious Diseases