P.Mean: What does "population-based" mean? (created 2008-09-12).

I was reviewing an article that I am a co-author on and the author described the dearth of "population-based" studies. I had to think a bit, about what that term really meant.

I had one guess, which was that a population based study was a study that carefully selected the patients so that they would be representative of an important population of patients. That's not at all accurate, and a Google search quickly set me straight.

I was set straight by one of the first articles I found:

Two sentences in the abstract show exactly what a population-based study is:

We conducted a retrospective cohort study of all children born in Denmark from January 1991 through December 1998. The cohort was selected on the basis of data from the Danish Civil Registration System, which assigns a unique identification number to every live-born infant and new resident in Denmark. MMR-vaccination status was obtained from the Danish National Board of Health. Information on the children's autism status was obtained from the Danish Psychiatric Central Register, which contains information on all diagnoses received by patients in psychiatric hospitals and outpatient clinics in Denmark.

A population-based study makes a serious effort to gain access to every single patient in a specified population.

But wait a second, what about

This abstract describes the study as follows:

Here, we investigate how exercise is associated with cognitive change and mortality in older people and, particularly, whether exercise might paradoxically increase the risk of dementia by allowing people to live longer. In the Canadian Study of Health and Aging (CSHA), of 8403 people who had baseline cognition measured and exercise reported at CSHA-1, 2219 had died and 5376 were re-examined at CSHA-2.

I don't think that 8,403 Canadians represent every single older patient. A review of the details of CSHA show that this indeed is not the case:

The CSHA involved 10,263 people aged 65 or over, sampled from 36 communities across Canada. Representative samples were drawn from the community and from institutions, and participants were assessed at 5-yearly intervals: in 1991, 1996, and for a final time in 2001. www.csha.ca/about_study.asp

So maybe I was right after all.

No but wait, what about this article?

An excerpt from this article says that

We family physicians increasingly are being asked by patients, fellow physicians, health plans and regulatory bodies to demonstrate objectively that our care really does make a difference and that our effectiveness extends to the entire population we care for, regardless of whether we've actually seen those patients recently.

So this means that a population based study is one that includes patients who are not seen regularly. A population based study does not exclude patients who are outside the normal scope of health care (it does not exclude, for example, patients who lack a primary care physician).

So maybe the Google search didn't help all that much. I'm more confused than ever.

In general, when you introduce a technical term, you should define and/or reference it, to avoid confusion. Perhaps I'm the only one who would be confused by this term (I hope not). But it is better to make fewer assumptions about how people interpret these terms.

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