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Dennis Lendrem
Dennis Lendrem
Making Data Make Sense
Posted in Business / Industries / Pharmaceuticals

Spiegelhalter’s Law & Health Risks

Feb. 14, 2012 1:00 pm
Doubling a very small number doesn't make it much bigger. Twice almost-nothing is still almost-nothing.
Spiegelhalter's Law

Bacon sandwiches are big news right now in the UK. A staple of the English diet,recent studies demonstrating increased cancer risks associated with eating bacon sandwiches have dismayed the great British public.

Sadly, the quality of the reporting surrounding this research has been, at best, rather patchy. For relative risks offer rich prospects for the hack journalist.

For example?

A study suggests that a bacon sandwich a day increases your risk of bowel cancer by 20 percent. Scary stuff! (This is particularly worrying news for those living in my corner of the world. Here, in Northumberland, we indulge in the Northumbrian bacon buttie - a potent mix of bacon, jam (that's jelly to our transatlantic cousins) and bread.)

Time to drop the beloved bacon buttie from the menu?

Not just yet. For to evaluate this risk we need to know the background rate of bowel cancers. Of 100 people we might expect 5 to get bowel cancer anyway. So, a 20 percent increased risk means the five in 100 people we would expect to get bowel cancer anyway will go up to six. In other words just one in 100 will be harmed in this way. So maybe the odd bacon buttie isn’t going to kill us after all.

Whenever you see a relative risk in a newspaper headline you need to start looking for the background risks. If they’re not reported then the chances are they’re quite small. The use of relative risks -- 20 percent increase, 30 percent decrease and so on -- makes effects look bigger. It's an example of the "framing" of a risk. Unscrupulous journalists manipulate the frame to sensationalise health risks.

Always remember that even if you double a very small number it isn’t going to make it much bigger. Twice almost-nothing is still almost-nothing, Spiegelhalter’s Law of Small Risks!

Never have bacon sandwiches so exercised the British nation.

More on risk from David Spiegelhalter

Mike Sutton
February 29, 2012 at 8:14 am
But what if you want to live forever

Hi Dennis

I suppose that given that a number of apparently rational scientists are beginning to speculate that given the emergence of new therapies that death through aging may no longer be inevitable for people born after 1955 (see the last edition of The Skeptic) then I suppose that we who wish to live forever might need to take account of the fact that small risks - over long enough time - become inevitabilities.

But, I suppose, this point ignores the issues that, (1) even accounting for high fat in the diet - and the harmful nitrates in cured meat such as bacon - that cholesterol levels in the bloodstrem - and bowel cancer- may have a genetic marker. And (2) new therapies might be able to deal with such diseases.

I suppose, in the round (no pun intended), until we know what can be done in the long term for clogged up arteries and bowel cancer that the best bet for those of us hoping to cash in on the predicted new therapies to get the maximum returns on their pensions investments (by this I mean getting one over on the grim reaper for longer than the insurance actuaries allowed for) is to lay off the bacon butties. Darn it!

And I agree with the last commentator – jam and bacon ….whatever next. Deep fried Mars bars I suppose. Perhaps such unhealthy yet tasty food should be on future top ten countdowns of the last things to try by those who thought they were not going to die? They could make that a reality TV show in the next century perhaps? Who knows if I manage to avoid eating bacon I might be around to watch or - worse still - partake in it.

Iain Grant
February 20, 2012 at 2:01 pm
Your Spiegelhalter analysis

I would like to hold up a mirror to your bacon sandwich metaphor of Spiegelhalter's Law.

You explain with admirable clarity (and correct sums:)) the fallacy of accepting relative risks as absolute. I applaud your consequent, implied denigration of sensationalist journalism (indeed, I am impressed by your restraint).

HOWEVER, (and I quote you verbatim)

Here, in Northumberland, we indulge in the Northumbrian bacon buttie - a potent mix of bacon, jam (that's jelly to our transatlantic cousins) and bread.)


jam? JAM??

dominus dimitte !!!

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