In medical research, the best way to test a new treatment is the randomized controlled trial: A number of people, all more or less equal in terms of disease or health, are randomly assigned to get a real treatment or a convincing fake (a placebo), or sometimes nothing at all.

It’s a great — but not perfect — way of finding out what works.

Were there more deaths in one group than in the other? Did one group have more heart attacks or cases of cancer? Is one group all-around healthier?

If the answer to one (or more) of these questions is: “Yes, this group is different from that one,” the researchers can be fairly confident the experimental treatment is the reason.

These kinds of experiments are hard enough to do when the subjects are people. They’re a lot harder when whole towns, counties and cities are assigned to get one treatment or another. But that’s what a few researchers have done periodically over the last 40 years.

Called community-based or population-wide trials, the studies were extremely influential in 20th-century medicine even though, ironically, many showed only modest effects.

Community-based studies are built on two insights made by epidemiologists trying to understand the epidemic of heart disease that emerged in many Western countries in the 1940s.

The first insight is that entire populations can differ from one another in their risk for certain diseases — something the English epidemiologist Geoffrey Rose described in 1985 in a famous paper called “Sick Individuals and Sick Populations.” A striking example was the comparison of cholesterol levels in southern Japan and eastern Finland. A graph showed that people with the highest cholesterol levels in Japan matched the lowest levels in Finland — one reason deaths from heart attack were far less common in Japan than in Finland.

The second insight is that if one were able to move an entire population in a healthier direction, more lives would be saved than if just the high-risk members were targeted for change.

Finland’s success

The most famous study based on these ideas was carried out in North Karelia, a province of 180,000 people in Finland. At the study’s start in 1972, North Karelian men had the highest heart attack rate in the world. Their average cholesterol count was 268 and blood pressure 149/92; 52 percent smoked. For local lumbermen, a common midmorning snack was a sandwich of pork fat sprinkled with salt.

Health authorities launched a massive public education campaign in North Karelia to restrict smoking and get people to quit; to eat less saturated fat, whole milk and dairy products; to eat more vegetables and less meat. They compared what happened there to a “control” province called Kuopio.

The risk profile of North Karelians immediately began to fall. The effects were such that the Finnish government rolled out the interventions — and eventually expanded their scope — to the whole country. More-stringent tobacco control measures were put in place; milk was repriced so that high-fat milk was more expensive; the first domestic vegetable oil industry (canola) was started.

The entire Finnish population became healthier, which made the progress in North Karelia seem less dramatic. Nevertheless, by 1997 death from heart disease had fallen in the province by 73 percent, compared with 65 percent in the whole country. Stroke and lung cancer rates also declined more steeply.

U.S. efforts

In the 1970s and 1980s, similar studies were tried in numerous communities in the United States. In general, the residents of the places that got media campaigns, classes and counseling in healthier living ended up with better risk profiles than residents in the “control” towns. But the differences weren’t big, and to some minds not worth the effort.

“The U.S. programs were certainly not an overwhelming success,” said Henry Blackburn, a physician and epidemiologist at the University of Minnesota who led some of these efforts. “But it probably wasn’t due to anything wrong with the hypothesis. It simply proved impossible at the time to do more than what was already happening in a dramatic way in the society at large.”

Despite their tepid success as experiments, few experts doubt the power of community-wide interventions. They’re a central — and growing — tool of federal efforts to combat smoking, obesity, diabetes and heart disease.

More are on the way, funded by the Obama administration’s health-care overhaul law and the economic stimulus package.