Pubdate: Wed, 4 Nov 2009
Source: Times, The (UK)
Page: 31
Copyright: 2009 Times Newspapers Ltd
Contact:  http://www.timesonline.co.uk/tol/news/
Details: http://www.mapinc.org/media/454
Author: David Spiegelhalter
Note: David Spiegelhalter is Winton Professor of the Public 
Understanding of Risk at Cambridge University

AN UNCERTAIN SCIENTIST'S GUIDE TO TAKING RISKS

Be cautious about invoking the 'precautionary principle'. We can never
be sure how dangerous - or safe - cannabis really is

Last year 509,090 people died in England and Wales, and the Office for
National Statistics (ONS) has just released full details of what they
died of. For a statistician this means 346 riveting pages of morbid
detail, ranging from the rare fatalities from hang-gliders (2), dog
bites (4), lightning (0, down from 2 the previous year),
men-in-their-40s on playground equipment (1) to the usual blockbusters
such as ischaemic heart disease (76,985).

Twelve riders were killed after falling off their horse in 2008, the
consequences of what David Nutt once called "equasy" - or the
addiction to horse-riding. The sacked chairman of the Advisory Council
on the Misuse of Drugs got into trouble earlier in the year for
comparing the risks of equasy with Ecstasy, which directly led to 27
deaths in 2006.

Since both have similar numbers of participants my guess is that
Ecstasy pips horse-riding in the risk stakes by a length. But given
that around 1,000,000 Ecstasy tablets are taken a week, these are not
high risks compared to the effects of alcohol, and certainly not other
Class A drugs with which Ecstasy is lumped.

In 2006 heroin was mentioned on 713 death certificates, and the
British Crime Survey estimated that 41,000 people used heroin that
year - this produces a (very) crude annual death rate of 1 in 58. Put
another way, heroin users have roughly the same death rate as an
average 65-year-old man or 71-year-old woman. You would have to go
hang-gliding eight times a day, all year, to have a similar risk.

But measuring the harm of drugs is not just about official statistics.
Most of us have our own stories about sad changes in young people that
appear closely associated with cannabis, and Professor Nutt points out
that users do have increased risk of psychotic episodes. Parents are
naturally concerned about the effects on the mental health of their
children, and although surveys show that cannabis use has fallen among
11 to 15-year-olds, and schizophrenia rates have also fallen, these
will provide little reassurance.

The ACMD acknowledges this and produced a ranking of the relative
danger of drugs based on nine aspects of harm, including the broader
social consequences.

The problem came when this harm ranking placed Ecstasy and cannabis
below alcohol and tobacco. Although Professor Nutt claims these legal
drugs can make useful benchmarks that help public understanding, the
arguments concerning lethality and legality get hopelessly entangled,
so that it is considered improper to "calibrate" illegal activities
against legal (and in the case of horse riding, admirably wholesome)
pursuits.

Jacqui Smith, the former Home Secretary, used public concern as part
of the basis for rejecting the council's recommendation to downgrade
cannabis, and also cited "doubt about the potential harm", saying "we
must err on the side of caution". As Professor Nutt pointed out, this
is a form of the precautionary principle, which says that if there is
a possibility of severe outcomes we should not wait for complete
certainty before acting.

This idea has been applied to man-made climate change, where if we
waited for full understanding it would be too late to do anything
about it. But the precautionary principle should be invoked with great
caution: otherwise every claim of possible harm, from autism after the
MMR jab to brain tumours from mobile phones, would lead to government
action - and careful weighing of evidence would be washed away in the
rush to bring about the illusory goal of "safety".

This means that the crucial role of any scientific advice is to assess
and communicate reasonable uncertainty, rather than just list what
"might" happen. Which is why it is so bizarre to have A. N. Wilson in
the Daily Mail caricaturing scientists as "arrogant gods of certainty".

Perhaps the author of this wonderfully ill-informed comment would like
to examine the recent UK Climate Impact Projections? There he would
find uncertainty by the bucket-load: projections allow for doubt about
how climate system works, accuracy of computer models and so on. A
wide distribution of possibilities is given, with even a small
probability that average temperatures will not increase at all.

The main problem is that if scientists and politicians are too
certain, then adapting to new information can be slow or embarrassing.
In 2007 the National Heart Forum made a projection that 48 per cent
(uncertainty range from 40-57 per cent) of girls aged 2-11 would be
overweight or obese in 2020. Yesterday, based on an additional three
years data, it radically reduced the projected proportion to 27 per
cent. How can this happen? The problem is that the projections are
based on a simple extrapolation of the data up to 2020, and so the
uncertainty limits do not allow for our doubts about what is going on
in the population.

It's only by acknowledging the "unknowns", and trying to quantify how
big they are, that scientists can give balanced, and appropriately
uncertain, advice.

Politicians need to be willing to accept scientific uncertainty and
still take decisions, and this means getting an idea of the magnitudes
of the risks, even when our understanding is incomplete. None of us
expects certainty in our lives, but we could all get better at
acknowledging our ignorance without succumbing to the twin perils of
panic or paralysis.
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MAP posted-by: Richard Lake