This is very informative, and I thought you might like to read it.

7 medical research terms you need to know

Angela Mulholland, News Staff

Date: Monday Sep. 6, 2010 7:22 AM ET

Why is it that health studies often seem to contradict one another? One day we're told our coffee habit is bad for us; the next we're told it might actually make us live longer.

The answer often comes down to how medical studies are performed. Some types of study are simply considered more reliable, which is why the advice from our doctors can change with the emergence of new studies.

So what is the best kind of study: a prospective, longitudinal, observational study, or a randomized, double-blind, controlled trial – and what the heck do any of those words mean anyway?

Here are a few of the terms that appear every day in medical journals, and what they mean to you.

Observational study
This is a type of study in which people or patients are watched over time to see how a particular treatment or behaviour affects them – for example: the effect of drinking wine over many years affects their risk for a heart attack.

An observational study is considered somewhat less reliable than a controlled experiment, because it cannot prove that a behaviour or treatment causes a result; it can only notice a link or patterns; hence the phrase "correlation does not imply causation."

But observational trials have their strengths. For example, they're indispensable when testing the effect of something over the course of decades, and/or over large groups of people. They're also a key study method to look for a side effect that crops up only rarely. In both cases, a controlled experiment would neither be practical nor feasible.

Prospective observational study
A prospective observational study, also called a "cohort study," is one type of an observational study.

This type of study follows a group of patients, called a "cohort," over time, gathering data from the time the study begins and then observing the effects over time.

A cohort study is often used to measure a number of behaviours and their effects over many years.

The Framingham Study is a well-known, long-term study looking at what causes heart attacks in people living in the town of Framingham, Mass. It began in 1948 and continues to this day. Much of what has been learned in the last half-century about how diet, exercise, smoking and cholesterol affect the heart came out of this study.

Retrospective observational study
A retrospective observational study, also called a "case-control study," looks at outcomes that have already occurred before the study has begun, and then looks back to gather evidence. So if a group of people is developing a similar type of cancer, researchers might match them with people who do not have the cancer, and then collect data to figure out what characteristics distinguish two groups.

Since retrospective studies tackle problems backwards, they are prone to some kinds of errors and are not always ideal. Indeed, the conclusions of many case-control studies have later been contradicted. But retrospective studies are often the first step in noticing patterns.

The conclusion that smoking causes lung cancer first began with retrospective studies, in which doctors noted a link between a rise in lung cancer rates and rises in smoking rates.

That led to prospective studies of smokers and how they developed lung cancer over time, and then to controlled experiments using mice exposed to smoke.

Longitudinal study