Panic Nation - Chapter 2: The misuse of numbers - Part 3

Continued from 21.6.2006

Stanley Feldman and Vincent Marks

Leaving aside the fact that this ideal weight is entirely subjective, it is self-evident that those who are only a few ounces, or even a few pounds, over the 'ideal' weight run a relatively small risk of any adverse effects on their health; it is those who are much heavier who are at risk of their lives being shortened by obesity. But the statistic gives us no idea of how many children are slightly overweight and how many are seriously overweight. What is does do, on the other hand, is serve to scare us and to sell newspapers.

Circumstantial evidence

Many of the scare stories that appear in the media start with an epidemiological study. These compare the rate, occurrence or coincidence of two events. Some of these studies are very good, with great care being taken to prevent bias and to limit the effect of outside factors.

Nevertheless, they all end up trying to demonstrate an association between a potentially causative factor and an effect. The problem with these sorts of studies, however, is that by itself the association between the events is circumstantial and says nothing about their causal relationship to one another.

Take the example given by Darrel Huff in his book How to live with statistics the greatest number of suicides in the UK occur in June; June is also the most popular month for marriages. Does this mean that these two events are related?

Bias

Many studies depend upon the reports of individuals. The results of such studies are often subject to bias, particularly when they are based on telephone or postal questionnaires. If a study trying to find out if a particular detergent caused skin irritation was carried out by post, it is likely that someone not troubled by skin irritation would throw the letter in the bin, whereas someone with skin irritation would reply.

Unless there was a very high return rate, the results would be subject to bias and be meaningless. A recent postal questionnaire sent out by the Home Office reported that 1 in 20 women had been raped. This result in intuitively unlikely, and the nature of the questionnaire was bound to give the result it did. The chances of happily married women who have never been raped replying to the questionnaire were very small.

In addition, if the definition of rape was not well defined, it is not beyond the bounds of possibility that some women, perhaps those in an unhappy relationship, will have answered the question 'Do you believe you have been raped?' rather than 'Have you been raped?' which is very different. The disturbing aspect of this study was not just the waste of taxpayers' money, but the fact that the results are likely to be used as the basis for legislation.

Telephone studies are just as likely to be subject to bias, as the respondent invariably tries to give the questioner answer that they think he or she wants to hear.

Unbalancing the equation

Benjamin, age five, casually remarked, 'God eats a lot of fish.' The background to his improbable assertion was his mother's remark that fish is good for your brain. His tendentious reasoning was that 'God must be very clever because he can hear what we are saying no matter what language we use', it follows therefore that 'he must eat a lot of fish'. A byline in the Sunday Times (August 2004) indicated that salt causes intellectual failure.

It appears an investigation in Boston showed, in a trail of 2,500 people, that high blood pressure was correlated with minor strokes, and minor strokes could cause intellectual degeneration, which is not unreasonable. The author, having believing that salt can cause high blood pressure wrote up the story to implicate salt as the cause of intellectual degeneration.

This piece of tendentious reporting ignores the evidence that the Japanese, who eat twice as much salt as an American on average, are no less clever and live longer lives.

Then there are numbers apparently picked out of the air but repeated so many times that they develop an authority of their own.

A letter from Transport for London says that 20,000 people die each year from atmospheric pollution in London. In fact, pollution in London is now lower than it has been for centuries. It is possible that a few with severe respiratory diseases are made worse by diesel fumes from lorries and buses but these are patients on the edge of breathing failure who would not be made better even if they lived in a plastic bubble fed with filtered air.

These figures are frankly false but because they go unchallenged become accepted as fact. With all the numbers that are bandied about to show this disease or that food is killing us we should remembers that we are living longer, healthier lives and that 'life itself is a fatal disease' if we do not die of one thing it will increase the death rate from an alternative.

Stanley Feldman is Editor Journal Anaesthetic Pharmacology Review, contributor Encyclopedia Britannica, Vincent Marks is a former president of the Association of Clinical Biochemists and erstwhile Vice President of the Royal College of Pathologists. He is one of England's best-known nutritionists.