Sunday, January 24, 2010

Populations and Individuals

One of the hallmarks of public health is that unlike medicine, you're thinking at the level of a population, not an individual.  This is one of the first things I learned about public health, but I feel like I still don't have a sophisticated understanding of how to think about the relationship between the two.

There are two aspects of this split that I'm thinking about: 1. how to intervene, how to avoid just telling collections of individuals the same thing a doctor would tell an individual (e.g. "eat less salt"); and 2. how to understand population level outcome measurements, how to make those statistics more meaningful than "you have an x% chance (based on data about other people like you and what happened to them) of getting this disease."

The NYT ran an article this week about salt reduction.  The inspiration for the article is a NEJM article which I have not read yet on the same topic, specifically with reference to the reduction of cardiovascular disease.
This sentence from the NYT article caught my attention:
"If everyone consumed half a teaspoon less salt per day, there would be between 54,000 and 99,000 fewer heart attacks each year and between 44,000 and 92,000 fewer deaths..."
This made me laugh because it reminded me of some sort of motivational group effort, like, if only we each did our part, now Get It Together, People!.  But half a teaspoon less salt per day is a LOT less salt.  Depending on gender, that would be a 30-40% reduction for the average American adult.  Small wonder that you'd see such big results.  I can totally believe that for a given individual, that reduction in salt could have dramatic effects on his or her health.

Now suppose it claimed, instead (not suggesting this is equivalent or true! thought experiment in progress): "If everyone consumed one pinch less salt per day, there would be between 540 and 990 fewer heart attacks each year and between 440 and 920 fewer deaths."

In that scenario, it seems less plausible that the good benefits are because any given individual who was eating x amount of salt is now eating (x - 1 pinch) of salt.  That is, it seems harder to interpret the sentence as being strictly true.  Put another way, if we had a laboratory of humans who were all fed exactly x grams of salt per day, and then we cut their salt intake to (x - 1 pinch) per day, would you see the results above?

Or: does the sentence really mean the following: some number of people (perhaps those who were eating way too much salt) suddenly dropped their salt intake dramatically (and the rest of us didn't make any changes), so that the average came out to each person eating 1 pinch less per day.  Then, those people who dramatically reduced their intake made up the group who didn't get heart attacks or die after all.

There are probably things I should already know that address this question.  For example, more than the averages are important, and standard deviations will address the question of variance in individual levels.  Furthermore, the basic split between longitudinal and cross-sectional studies addresses these concerns (hey, I learned something in my speech-language pathology class!).  I know there are aspects of cross-sectional study methodologies that try to overcome the effect of all the averages (average of day-to-day salt intake per individual, average of individuals' salt intake to give population level estimates).  Maybe after I read the actual study I'll have a better idea of how it works, and whether the NYT quote above is a misrepresentation or not.

Final notes on the NYT article.  Kudos for:

1. acknowledging that action beyond the level of the individual might be appropriate
2. quoting someone on the idea that when we legislate these kinds of things, we must be very wary of unintended consequences
3. mentioning some subpopulations that would particularly benefit (would have gotten extra points for mentioning health disparities more broadly)
4. quoting the study author on the population-individual issue

Boos for:

1. being wishy-washy about where the bulk of the sodium intake is from. As far as I know, the research clearly points to processed and restaurant foods
2. having a kind of pointless quotation about how instead we should be working on anti-smoking efforts because they'd make a bigger difference.  Who said we should put all our energies toward one single problem? This just seemed irrelevant, like the reporter simply wanted one more dissenting opinion

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