Tuesday, 12 July 2011

Smoking horror statistics - missing figures round

From the BBC:

Women who smoke while pregnant should be aware that they are increasing the chance their baby will be born malformed, say experts. The risk for having a baby with missing or deformed limbs or a cleft lip is over 25% higher for smokers, data show.

Along with higher risks of miscarriage and low birth weight, it is another good reason to encourage women to quit, say University College London doctors. In England and Wales 17% of women smoke during pregnancy. And among under 20s the figure is 45%.

Although most will go on to have a healthy baby, smoking can cause considerable damage to the unborn child. Researchers now estimate that each year in England and Wales several hundred babies are born with a physical defect directly caused by their mother's smoking. Every year in England and Wales around 3,700 babies in total are born with such a condition.


OK, 3,700 babies are born with defects out of 750,000 babies a year. We can minus off a third of those caused by Pakistanis marrying their first cousins, which gets it down to a third of a per cent with defects, call it 2,500 a year.

Question 1: If 17% of women smoke; the overall average risk of a baby born to normal couple having a defect is 0.33%; and the risk of your baby having a defect is 25% higher if you smoke (they cherry picked diseases where the risk is measurably higher, and didn't mention all the defects where the risk is the same whether you smoke or not, but hey), then what are the chances of having a baby with a defect if you:
a) Don't smoke during pregnancy.
b) Smoke during pregnancy.

Click and highlight to reveal answers: a) 0.32%, b) 0.4% (you can guess this answer, one is 0.33% minus a tiny bit and the other is 0.33% plus five tiny bits).

Question 2: How many babies born to mothers who smoke will have birth defects?

Click and highlight to reveal answer: 750,000 babies x 17% smokers x 0.4% with defects = 510 babies.

Question 3: If those mothers hadn't smoked, how many of their babies would have birth defects anyway?

Click and highlight to reveal answer: 750,000 babies x 17% x 0.32% = 408.

Question 4: Deduct your answer from 3 from your answer from 2 to calculate the additional number of babies born with defects as a result of the mother smoking?

Click and highlight to reveal answer: 510 - 408 = 102. It's not "several hundred", is it? There's a wide margin of error here, but I'd politely refer to that as "one hundred", i.e. a tenth as many as babies born with defects due to inbreeding.

16 comments:

Sue said...

I smoked during both my pregnancies and had a pint of stout every evening too. I couldn't take iron tablets, they gave me migraine, so I had to acquire the taste for Guinness (My Irish Doctors orders).

I had small babies but I was only 7 stone myself when I got pregnant. They're both taller and bigger than me now. One's a smoker, the other isn't.

They should be worried about drug addicts and inbreeding first rather than this constant war against smokers.

I would have given up years ago if I hadn't been constantly nagged and treated like a leper in the first place.

Mark Wadsworth said...

S, glad to hear that you were one of the lucky 99.6% of smokers who have healthy babies :-)

"I would have given up years ago if I hadn't been constantly nagged and treated like a leper in the first place."

I do wonder sometimes whether the same applies to me. My basic rule is "Do and say the opposite of everybody else on the off-chance they are wrong."

Hopper said...

Aren't your answers to Q.1 the wrong way around? Other than that, spot on.

Mark Wadsworth said...

H, oops, well spotted, I have amended.

dearieme said...

Here's a merry wheeze for you to try, Wadders. Gather ye some numbers and analyse them and you'll find - I conjecture - that wome who smoke while pregnant have parents who have an increased chance of being malformed. I'll bet it runs back up the generations too. Betcha.

Dick Puddlecote said...

Nice analysis, MW. It's how all public health pleading tends to go. Present the biggest number possible.

25% of a tiny percentage is an even tinier number, so they don't use that one.

And to think these guys have the gall to lump themselves in with the public health of yore who ushered in sanitisation and clean water (yes, they really do class themselves as that important).

Anonymous said...

Excellent pointing out of the numbers and proves the point exactly. The societal shame is that BBC would report what was claimed copy/paste/sound-bite style with no questions asked. Here-in is the analysis, but the shame being BBC won't pick up on it and publish these facts, to be fair. Maybe it's fodder for a good book, find enough of these that can be mathematically debunked.

Anonymous said...

Being over 35 appears to be a far bigger risk for birth defects than smoking.

Second,this study is a meta analysis - a pooling of many different studies carried out on different populations with perhaps different criteria for recording defects. Each study may have attempted to correct for different sets of confounders eg alcohol consumption (notoriously difficult when pregnancy and smoking are involved), urban air pollution. But with such a low relative risk (1.25) hidden confounders can easily render the results invalid. For example, why do some women smoke? Is it a form of self medication - connected with depression? Are smoking women under more stress? Who knows?
And Sue, I might have given up some time in the last four years; but now, no chance. This is a war. I hope to see the day I'mm sitting in a pub smoking a cigarette. Death or glory. Don't ever give up the fight.

Sue said...

Nope, I'm not giving up. I really hate being told what I can and can't do!

I actually ENJOY smoking. I even smoke my own marijuana which I grow on my terrace here in Spain, it's lovely :)

Mark Wadsworth said...

D, you're the scientist, you work it out.

DP, ta. This is of course nothing to do with 'public health' as you rightly point out.

Anon(s) yes, ta for back up info.

S, yes i also enjoy smoking, else why would i do it?

Anonymous said...

I lost my temper Somewhere Else on this and had a go at Professor Hottentott for overlooking [0]the shallowness of the Pakistani gene pool, when he felt that he had to break it to the world that smokers will give birth to mutants.

Someone called Allan Hackshaw has written a book called 'how to write a grant application'. It seems he also participated in this circle jerk: http://www.publications.parliament.uk/pa/cm200506/cmselect/cmhealth/485/5102005.htm . Excellent reading if you'd like to lose some weight.

[0] well, he would, wouldn't he, having seen a picture of him

Mark Wadsworth said...

Anon, that link really is the Motherlode Of Shite, it is quite clear that the "deaths from passive smoking" figure is

a) completely plucked out of the air, and

b) probably wildly overstated at 12,000 a year in the UK - that's one in forty deaths, for crying out loud.

Junican said...

The lady said more or less directly that the fugures are based upon actual smoking figures. She also said that it is 'a formula' which produces the figures. I wonder what the computer formula was?

A lovely verification word - cro -bias!

SadButMadLad said...

Another view on stats and how they are used to lie - www.annaraccoon.com/reflections/risky-business/

Derek said...

And since we're on the topic there is an excellent little book from the 1950s, called How to Lie with Statistics by a guy called Darrell Huff. It's an excellent easy read and will alert you to all sorts of tricks. No home should be without it.

Anonymous said...

"I would have given up years ago if I hadn't been constantly nagged and treated like a leper in the first place."

I started smoking about five years ago, when the hysteria about national no smoking day reached its peak at my workplace.