Percentages might make news stories stand out, but without a connection to the human experience, can become meaningless. Wouldn’t it be good to have the mental agility to separate the wheat from the chaff? In his third lesson of a weekly series, author Michael Blastland gives some hints for percentages.
Lesson Three: Percentages
The story: Vitamin E can kill. Supplements cause a 14% increase in mortality, said the man on telly, standing in front of a huge “14%”, propped up on a number like the bar of the local.
The flaw: Funny, but wasn’t my risk of mortality quite high already? Like, 100%? But no, it’s even worse than that. Vitamin E supplements apparently make the end more than certain. For death has become like a footballer, giving it 114% out there today, Brian.
The lesson: Journalists and percentages mix like ball bearings in souffle. They say their preference is for real life over boring, abstract figures. Not with percentages, it’s not.
Often these digits become no more than that: digits, fetishised, but separate from human meaning and, in the case of risk, frequently reported only if they can be made to look big or scary.
We need to reconnect them with human experience. Here are two ways.
First, we need to remember that not much in life is either/or. According to the research, there’s something in the claim that Vitamin E supplements can be harmful. But, as with the consumption of salt, or even water, much that can kill is also essential to good health.
The world does not divide easily into what’s toxic and what’s not, what’s safe and what isn’t. Risk is simply a way of measuring where we stand on the messy middle ground - which is almost everywhere.
What matters in that messy middle is the relevant human quantity: how much supplementary vitamin E? A little won’t do any harm (or, probably, much good). A lot, especially if you are getting on in life, might.
So a 14% increase in risk of death does mean something, but only if you say at what dose (high), for which group (the elderly), over what period (a single year, not in a lifetime).
The second common problem with any percentage increase like this, also crying out for a dose of real life is: what’s it increased from? Because 14% might be a lot if you start somewhere big, next to nothing if you start somewhere small.
A 100% increase from one in a million becomes two in a million. So what?
A 100% increase in the number of bullets in a revolver - if you are playing Russian roulette - well, that makes a difference.
How can supplements cause a 14% increase in mortality?
So let’s get human again and ask where we begin. What is the risk of dying, this year, for a 75-year-old man? And let’s keep percentages out of it, as far as we can.
The answer is that in every 100 men aged 75, four or more will typically die in the next year. If all 100 of them tuck heartily into Vitamin E, maybe five will.
That is, according to this research, there’ll be less than one (actually 0.6) extra death per 100 people if all are on high doses of vitamin E. At low doses, there’s almost no change.
That’s what 14% turns out to mean: 0.6 of a death per 100, or six deaths per 1,000. I wonder if a large daily dose of salt is any worse?
For 60-year-olds, the risk of death this year is so much smaller to begin with that even an extra 14% risk (from a high dose of Vitamin E) makes almost no difference at all.
Junk rating: Four out of five. There’s some salvation in the fact that this was part of a report on all kinds of teenage cancers, some of which are a real concern, but this was the one that had the attention. Yet the human numbers are so small they might well be the result of nothing more than random variation.
A little context can make a lot of difference to a percentage. Another simple example is the story that young Americans can’t afford to move out and are all now stuck with the folks.
Since 1970, when 12.5 million 18-34 year olds lived with their parents, the number has apparently risen by 48%, to more than 18.6 million.
Except that the American population has also risen in that time - by about 32%, or roughly 75 million people. So, in figures easier to translate to real life, the number of 18-34 year olds still living with the folks has gone from a bit more than one in five to not quite one in four. Still a real increase, but nothing like 48%.
The biggest part (though not all) of the explanation for increasing numbers of Americans living with their parents is… that there are increasing numbers of Americans.
Or take what it means to be 99.9% effective. In the NHS it might mean 135 botched operations and 13 babies given to the wrong parents, every week. (Thanks to writer Anne Miller for that example).
A percentage is not really a number, it is a share. The simple question to keep in mind is one that always strives to put it into a proper, human context: “A share of what? A share of a lot - or a share of a little?” Better still: “A share of who?”
Keep it real.
Next week, Lesson Four: Averages
Michael Blastland is the author, with Andrew Dilnot, of The Tiger That Isn’t.
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