Can something good for us be bad for you?

A few weeks ago, the media was excited to report that coffee decreases your risk of prostate cancer. Coffee is good for us! Yay!

In fact, we see these kinds of reports every day. That’s why WebMD is in business– to report on confusing esoteric things that are deemed good for us or bad for us. But let’s take this kind of report as an example. The following are just made up numbers for illustrative purposes.

Say they take 100 people and study them. At the end of a few years they find that, lo and behold, heavy coffee drinkers have less prostate cancer. Let’s say this good effect appears in 40 people. However, in 30 people there’s no effect at all. And the other 30 people, well, unfortunately, there was a mild increase in prostate cancer. To summarize:

  • 40 = mild decrease in cancer
  • 30 = no effect
  • 30 = mild increase in cancer

So the decrease in prostate cancer was seen in 10 more people than the increase. So, in the entire population of 100 people, coffee looks as if it’s good for that population. Therefore, the authors were happy to report that coffee is “good” for us.

But in reality, it doesn’t help 30% of people, and mildly increases risk in 30% of people. If you’re lucky enough to be in that 40% category, great, the “coffee is good for you” rhetoric applies to you. If you’re part of that other 60%, well, coffee may be doing nothing or even harming you. But, in the entire population, yay, coffee is good for us!

In fact, this is the fundamental method of studying modern medicine. This concept applies to everything– all the way from food to Lipitor.

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