Daily Mail or the daily espresso in the morning?

September 28, 2011 1 Comment

An article today on the BBC regarding research into coffee consumption and depression led me to look up what the Daily Mail had published. The BBC headlined with “Coffee may prevent depression”, whereas the Daily Mail opened with “Coffee is good for you“.

The BBC at least mentioned that the 50,000 study participants were all US female nurses and linked to the research article, though neither revealed that the mean age of those involved was 63. The Daily Mail also adds in a bit of science which is just plain wrong, “…caffeine works like antidepressant pills by stopping the production of certain hormones such as serotonin”.

Both give the figure that those who drink between 3 cups per day were 15% less likely to suffer depression without giving the absolute risk difference (there were only 2607 cases of depression total over the ten years in the study.) A rough, back-of-the-envelope calculation gives “Out of 10,000 quite old women, half of which drink coffee, there will be 27 cases of depression amongst the non-drinkers this year and only 24 within the coffee-drinking group.” I guess that’s why I don’t write headlines.

The BBC article mentions other factors in the study, and how the initial headline statistic might come down once they are controlled for. The Daily Mail does not. Neither article considers whether this small difference is casual or correlative (coffee drinkers perhaps being less depressed because the reason they drink coffee in the first place is because they have lots of friends to meet up with at Starbucks).

courtesy of xkcd.com

 

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Selection Effects

September 15, 2011 9 Comments

An article on the BBC website just now regarding the London riots has caught my attention with the headline:

One in four riot suspects had 10 previous offences

And it goes onto say that:

Three-quarters had a previous caution or conviction

Is it really true that 75% of those committing crimes on those evenings had previous convictions, or is it that those with previous convictions have their details on a police database and it was therefore possible to identify (and find) those people based on images from photographs and recordings made last month?

Many people fail to notice when a selection effect (selection bias) is occurring, and in many situations. I remember many weekend nights out in my youth with my good friend John where he would observe that all the girls were going in the opposite direction to us and should we not go somewhere else. I stuck to the line that we were only ever going to pass people going in the opposite direction. It certainly wasn’t anything to do with us being uncool.

And here’s a good article on World War 2, regarding selection bias and aircraft design. An engineer was asked to inspect aircraft returning from battle over a period of time, and to come up with a recommendation on where to add armour. After building up a statistical model of where the aircraft he inspected has sustained damage, he recommended reinforcing armour in the parts that were generally not damaged. Can you think why? Answer under the fold.

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