To synthesize what we have addressed so far with regard to reading medical literature critically:
1. Always identify the question addressed by the study first. The question will inform the study design.
2. Two broad categories of studies are observational and interventional.
3. Some observational designs, such as cross-sectional and ecological, are adequate only for hypothesis generation and NOT for hypothesis testing.
4. Hypothesis testing does not require an interventional study, but can be done in an appropriately designed observational study.
In the last post, where we addressed at length both cross-sectional and ecologic studies, we introduced the following scheme to help us navigate study designs:
Case-control studies are done when the outcome of interest is rare. These are typically retrospective studies, taking advantage of already existing data. By virtue of this they are quite cost-effective. Cases are defined by the presence of a particular outcome (e.g., bronchiectasis), and controls have to come from a similar underlying population. The exposures (e.g., chronic lung infection) are identified backwards, if you will. In all honesty, case-control studies are very tricky to design well, analyze well and interpret well. Furthermore, it has been my experience that many authors frequently confuse case-control with cohort designs. I cannot tell you how many times as a peer-reviewer I have had to point out to the authors that they have erroneously pegged their study as a case-control when in reality it was a cohort study. And in the interest of full disclosure, once, many years ago, an editor pointed out a similar error to me in one of my papers. The hallmark of case-control is that the selection criteria are the end of the line, or the presence of a particular outcome, and all other data are collected backwards from this point.
Cohort studies, on the other hand, are characterized by defining exposure(s) and examining outcomes occurring after these exposures. Similar to case-control design, retrospective studies are opportunistic in that they look at already collected data (e.g., administrative records, electronic medical records, microbiology data). So, although retrospective here means that we are using data collected in the past, the direction of the events of interest is forward. This is why they are named cohort studies, to evoke a vision of Caesar's army advancing on their enemy.
Some of the well known examples of prospective cohort studies are The Framingham Study, The Nurses Study, and many others. These are bulky and enormously expensive undertakings, going on over decades, addressing myriad hypotheses. But the returns can be pretty impressive -- just look at how much we have learned about coronary disease, its risk factors and modifiers from the Framingham cohort!
Although these observational designs have been used to study therapeutic interventions and their consequences, the HRT story is a vivid illustration of the potential pitfalls of these designs to answer such questions. Case-control and cohort studies are better left for answering questions about such risks as occupational, behavioral and environmental exposures. Caution is to be exercised when testing hypotheses about the outcomes of treatment -- these hypotheses are best generated in observational studies, but tested in interventional trials.
Which brings us to interventional designs, the most commonly encountered of which is a randomized controlled trial (RCT). I do not want to belabor this, as RCT has garnered its (un)fair share of attention. Suffice it to say that matters of efficacy (does a particular intervention work statistically better than the placebo) are best addressed with an RCT. One of the distinct shortcomings of this design is its narrow focus on very controlled events, frequently accompanied by examining surrogate (e.g., blood pressure control) rather than meaningful clinical (e.g., death from stroke) outcomes. This feature makes the results quite dubious when translated to the real world. In fact, it is well appreciated that we are prone to see much less spectacular results in everyday practice. What happens in the real world is termed "effectiveness", and, though ideally also addressed via an RCT, is, pragmatically speaking, less amenable to this design. You may see mention of pragmatic clinical trials of effectiveness, but again they are pragmatic in name only, being impossibly labor- and resource-intensive.
Just a few words about before-and after studies, as this is the design pervasive in quality literature. You may recall the Keystone project in Michigan, which put checklists and Peter Pronovost on the map. The most publicized portion of the project was aimed at eradication of central line-associated blood stream infections (CLABSI) (you will find a detailed description in this reference, Pronovost et al. N Engl J Med 2006;355:2725-32). The exposure was a comprehensive evidence-based intervention bundle geared ultimately at building a "culture of safety" in the ICU. The authors call this a cohort design, but the deliberate nature of the intervention arguably puts it into an interventional trial category. Regardless of what we call it, the "before" refers to measurement of CLABSI rates prior to the intervention, while the "after", of course, is following it. There are many issues with this type of a design, ranging from confounding to Hawthorne effect, and I hope to address these in later posts. For now, just be aware that this is a design that you will encounter a lot if you read quality and safety literature.
I will not say much about the cross-over design, as it is fairly self-explanatory and is relatively infrequently used. Suffice it to say that subjects can serve as their own controls in that they get to experience both the experimental treatment and the comparator in tandem. This is also fraught with many methodologic issues, which we will be touching upon in future posts.
The broad category of "Other" in the above schema is basically a wastebasket for me to put designs that are not amenable to being categorized as observational or interventional. Cost effectiveness studies frequently fall into this category, as do decision and Markov models.
Let's stop here for now. In the next post we will start to address threats to study validity. I welcome your questions and comments -- they will help me to optimize this series' usefulness.