Understanding Medical Research
What to Look for When Reading Medical Research
Reading medical news can be an exercise in frustration. Sometimes it seems that this month's findings always contradict last month's. How do you know what to believe?
Sometimes you don't. But one clue to quality is the design of the study. Although no study is perfect, some are more likely to yield reliable results than others.
The gold standard is the randomized controlled trial (RCT). This kind of study always has these features:
- It has a treatment group. These people receive the therapy under study.
- It has a control group. These people are as similar as possible to those in the treatment group except they do not get the experimental treatment. Instead, they may receive the standard treatment or a placebo (such as a pill with no active ingredient).
- Subjects are randomly assigned. Neither subjects nor researchers get to choose who goes in which group. Assignment is determined by chance, such as by tossing a coin or having a computer randomly assign the volunteers.
An RCT often has these good features as well:
- It is double blinded: Neither patients nor researchers know who is in which group. Blinding greatly boosts a study's quality. But blinding is not always possible—one example of this would be a study that compares patients who've had surgery with those who have not.
- It has a large sample size (number of subjects).
- Subjects are followed for a long time.
Other Types Of Studies
If RCTs provide the best evidence, why do scientists conduct other kinds of studies?
Cost, for one. RCTs are expensive; they usually must be funded with large grants from government or industry. They also take a long time. And in some cases, RCTs aren't even possible. For example, it would be highly unethical to expose a group of people to smallpox. So scientists must do other types of studies.
One step down from RCTs are case-control and cohort studies.
- A case-control study looks backwards. It compares people who already have a certain disease with a group of similar people without the disease to look for links between the disease and past events. For example, researchers might compare the foods eaten by cruise ship passengers who got food poisoning and those who didn't.
- A cohort study looks forward. It starts with people who've been exposed to a certain risk factor and other people who haven't and sees what happens to them later. For example, a study might follow women who've used hormones and those who haven't to track and compare rates of cancer development.
Other scientific studies are a notch lower in the strength of evidence they provide.
- A cross-sectional study looks at a single point in time. Examples include surveying people with diabetes to see how many take aspirin and comparing rates of arthritis in overweight and lean people.
- An ecological study compares rates of diseases in different populations—for example, cavity rates in towns with different levels of fluoride in their water.
- Case and case-series reports provide the weakest evidence. They describe the experiences of one patient (case report) or a few patients (case-series report). For example, when AIDS first appeared, so did case reports describing seriously ill patients with a puzzling cluster of symptoms.
- Another type of study gaining popularity is the meta-analysis. In this type of study, the scientist does not actually treat or observe subjects. Instead, the scientist searches the medical literature for similar studies and pools the results. In effect, a meta-analysis creates a study with a large sample size. A meta-analysis, however, is only as good as the studies it includes.