Front page image used under Creative Common License. Photo credit: Flickr user cherrysweetdeal.
Image used under Creative Commons License. Photo credit: Flickr user antwerpenR.
The blogs were full of it, the network and cable news channels were full of it, every online-news outlet featured it:
News outlets all over the world went nuts with it! “According to the study, drinking two or three cups of coffee a day reduces the risk of depression in women by 15%.”
Really? 15%? That is amazing! Anything that reduces the risk of depression by 15% ought to be a standard remedy. And just 2-3 cups of coffee? Wow! What better marketing for Starbucks.
But then upon reflection, it starts to sound like the “Three Glasses of milk prevents cardiovascular disease” bullshit.
Let’s look at this coffee-prevents-depression-in-women study a bit closer.
One of the most common mistakes–and the science, and the reporting of the coffee-prevents-depression-in-women item were certainly guilty of it, is the “association versus causation” distinction, that Schwitzer has written about.
It goes like this:
Say you are a scientist and have a group of 12 monkeys that were born in your lab. From their birth you make sure the monkeys wake up early in the morning. You keep this up until they grow old and die. Then you note that their average lifespan was 30 years. (The average lifespan of a monkey in the wild is about 25) So you write a paper that says that waking up early can increase your lifespan by 20%.
That 20% is a huge number! That means that a human life-expectancy of 78 years (average in the US) can be increased by an additional 15.6 years by waking up early.
But is that really correct?
Not it is not. Here’s why.
What happened was that you found and association between monkeys waking up early and their lifespan. But not a causation. You have no evidence to show that waking up early causes an increase in lifespan. There may be many more such associations apparent that you may chose to ignore. For example, the early-riser group of monkeys may have more of a violent disposition, but you may chose to not report that, or ignore it altogether. Maybe waking up early makes monkeys cranky.
In order to do further research you may have to have 2 groups of monkeys: A and B. You wake up the A group early and let the B group sleep as late as they like. Then you note if there is a difference in their lifespan. And even if there is, there may be still other factors that may be the cause of this seeming association. Science is hard and slogging. And rightfully so.
Now back to the coffee-prevents-depression-in-women study. They found an association in one study of nurses between coffee intake and lower depression.
But so what?
Here’s the other few associations and correlations they found in the same coffee-prevents-depression-in-women study—but the study ignores them:
[By the way, I am quoting here from from "Grab your Coffee, I think this paper may depress you" at the Scicurious blog at Scientific American.]
1) Smoking. The interaction between depression risk, smoking, and coffee consumption was “marginally” significant but they dismiss it as being due to chance because it was “unexpected”.
2) Drinking: heavy coffee drinkers drink more. But note that they don’t say that drinking coffee puts you at risk for drinking alcohol.
3) Obesity: heavy coffee drinkers are, on average, thinner, but not more physically active. They do not conclude that coffee drinking prevents obesity.
4) Church going: heavy coffee drinkers are less likely to go to church. Less likely to go to church, less likely to develop depression…heck, forget depression, maybe coffee prevents religion now! Now THAT would be a heck of a finding.
Boy, what would I give to see these headlines:
“Women who drink coffee likely to be heavier smokers.”
“Coffee drinking causes heavier alcohol use in women.”
“Coffee drinking prevents obesity in women.”
“Coffee-drinking women less likely to go to church.”
Do my headlines sound ridiculous? Of course they do. That’s what happens when you create “Health News” out of associations rather than causation.
So stop believing this coffee-prevents-depression-in-women kind of nonsense and educate yourself a bit.
Read more on the issue here at Scicurious blog at the Scientific American website.