Bayesian Critique of Statistics in Health
The Great Health Hoax
If you were going to have a heart attack. it seemed there was never a better
time than the early 1990s. Leading medical journals were regularly reporting
results from trials of new treatments for heart attacks that weren't just good
- they were incredible.
In September 1992. the British Medical Journal published results from trials
in Scotland of a clot-busting drug called anistreplase which suggested it
could double the survival chances of a heart-attack victim. The following
year another "miracle cure" emerged: injections of magnesium, studies
suggested. could also halve death rates. Leading cardiologists hailed the
injections as an "effective, safe. simple and inexpensive" treatment that
could save the lives of thousands.
But then something odd began to happen. In 1995, The Lancet published the
results of a huge international study of heart-attack survival rates among
58.000 patients -- and the "amazing" life-saving abilities of magnesium
injections had simply vanished. Anistreplase fared little better: the current
view is that its real effectiveness is barely half that originally claimed.
Other "amazing" heart drugs seem to have suffered the same fate, also
mysteriously losing their potency once on the wards. A study published last
year in the BMJ compared death rates among cardiac patients in the early 1990s
with those back in the early 1980s, in the Dark Ages before the advent of
"clinically proven" heart-attack treatments. What Dr Nigel Brown and his
colleagues at Queen's Medical Centre in Nottingham found was disconcerting:
"Despite an increasing uptake of the 'proved' treat- ments, in-patient
mortality ... did not change." The death rate in 1992 was the same as in 1982:
20 per cent - and double what had been found in the trials.
Scientists have invented a long list of excuses to account for the
disappointments. Some blame the fact that patients in clinical trials tend
to-be hand-picked and 'fussed over by leading experts. Others argue that
patients arrived on wards too late for the wonder-drugs to work. Or, perhaps
the original trials simply hadn't been big enough. But there is another
explanation, and its implications are so serious that scientists who
understand them often refuse to go on the record to talk about them. They have
been discussed behind closed doors by leading scientific journals and academic
institutions - only to be swept under the carpet. Meanwhile, millions of
pounds of taxpayers' money are being wasted follow-ing up illusory
At the centre of this scandal is a simple statistical technique used by
scientists as the basis for suppoosed research brekthroughs. It is called
'significance testing'. And it is fatally flawed.
When used to analyse clinical trials, significance testing can easily double
the apparent effectiveness of a new drug, and turn a borderline result into a
"significant" breakthrough. It can throw up convincing - yet utterly spurious
- evidence for links between diseases and any number of supposed causes.
But even more astonishing than these dangerous flaws is the fact that experts
have been warning about the methodology for more than 30 years - but the
scientific community has refused to act. In the meantime, a host of
implausible "breakthroughs", which raise the hopes of patients and
families. are announced with a fanfare . . . and then quietly disappear.
Take another case. In 1994. Dr Michael Mendall and colleagues at St
George's Hospital Medical School, London, made the claim that heart
disease was linked to a bacterium in the human stomach. Quite how a bug
in the stomach could possibly damage the heart was far from obvious. No
matter: the standard textbook statistical tests pointed to a
"significant" link. The existence of the link was confirmed by other
studies - again using the standard statistical tests. But last November.
a team at Bart's Hospital published the results of the biggest-ever
investigation into the link. And it had simply vanished.
A similar story surrounds claims that emerged in the mid-1980s that aspirin
could prevent pre-eclampsia, a potentially fatal condition that affects up to
one in seven pregnant women. By the early 1990s, many studies seemed to
confirm the theory, reporting a far lower rate of pre- eclampsia among women
given low doses of aspirin. Once again, no one knew why it should work -- the
cause of pre-eclampsia is unknown -- but the findings were still
"statistically significant". In 1994, however, the aspirin and pre-eclampsia
link went the same way as claims for stomach bugs and heart disease: a major
international study failed to find what the small studies had seen.
Health scares offer further evidence of the exaggerating effect of
significance tests. Claims of a link between living near pylons and leukaemias
in children have been made since the 1970s. The most impressive evidence
emerged in 1992. when a team from the well-respected Karo-linska institute of
Stockholm found a "highly significant" increase of three- to four-fold in the
risk of leukaemias. Again. quite why the risk should be so huge was unclear -
and yet again, when the biggest- ever study into the claim was published last
year, the link had evaporated.
Vitamin K injections and leukaemias; silicone breast implants and
connective-tissue disease; salt and high blood pressure; passive smoking and
lung cancer - the list of statistically based public "scares" goes on.
Just why has the scientific community failed to act? The answer lies in its
squeamishness about subjectivity. It is hard to convey the strength of emotion
aroused within the scientific community by the "S-word". Subjectivity is seen
as the barbarian at the gates of science, the enemy of objective truth. So
when, in the 1920s, the brilliant Cambridge mathematician and geneticist
Ronald Aylmer Fisher came up with an apparently objective way of drawing
conclusions from experiments, it was seized upon by the scientific community.
In 1925. Fisher published his techniques in a book. Statistical Methods
for Research Workers. It has become one of the most influential texts in
the history of science, and forms the foundation of virtually all the
statistics now used by scientists. Its methods are taught today. On the
face of it. Fisher had found techniques anyone -- sceptic or advocate --
could use to prove the significance of a new finding.
Critical to Fisher's method is the so-called P-value -- defined as the chances
of getting at least as impressive evidence as that actually seen if mere fluke
were at work. These P-values are worked out mathematically from the raw
experimentla data. According to Fisher, if the resulting P- value is below
0.05, then it is safet to label a finding "significant".
Combining simplicity and apparent objectivity, Fisher's P-value method was an
immediate hit and its popularity endures to this day. Open any leading
scientific journal and you will see the phrase, P less than 0.05 in papers on
every conceivable area of research, form astronomy to zoology. Indeed,
national governments, including ours, still use Fisher's standard to decide
whehter a new life-saving drug should be approved for use.
But no sooner were P-values taken up throughout the scientific community than
other statisticians began to ask some awkward questions. Most importantly,
just how did Fisher know that his figure of 0.05 was a safe point at which to
declare a result 'significant'? Incredibly, as Fisher himself admitted, he
didn't know at all. He simply chose the figure of 0.05 because, he said, it
The implications are stark. It means that vital scientific questions --
whether a new heart drug is seen as effective or whether breast implants
trigger disease, for example -- are being decided by an entirely arbitrary
The first hints of this appalling flaw in Fisher's methods emerged as long ago
as the early 1960s, following a resurgence of interest in a 200-year-old
mathematical formula known as Bayes's Theorem. Put simply, Bayes discovered a
mathematical recipe for working out how to update one's belief in a theory as
new evidence emerges. It was a fundamental discovery, for Bayes's Theorem gave
scientists a way of working out just how much more plausible their theories
become as the data rolls in.
There was a problem, however: Bayes's Theorem revealed that before any new
finding can be deemed 'significant', a crucial factor must be included: its
plausibility. Non-scientists may hardly regard that as a problem at all:
surely it makes sense to gauge just how plausible a result is before declaring
it a breakthrough?
But within the scientific community, Bayes's Theorem acquired a reputation for
being dangerously subversive. Its insistence on plausibility means that
different people can reach different conclusions about the 'significance' of
findings. And that has tainted Bayes's Theorem with that most repellent
concept known to scientists: subjectivity.
Regardless of how awful they found Bayes's Theorem, however, scientists could
not evade it. And its implications for Fisher's P-values turn out to be grave
indeed. In the 1960s, at the University of Michigan, a team of statisticians
including Prof Leonard Savage -- one of the most distinguished experts on
probability -- showed that P-values were easily capable of boosting the
apparent significance of implausible results by a factor of 10 or more. They
went on to issue a warning that P-values were 'startlingly prone' to attribute
significance to fluke results.
Despite being published in the prestigious Psychological Review, the warning
went unheeded. Over the next 30 years, other statisticians also sounded the
alarm bell, again without effect. During the 1980s, Prof James Berger of
Purdue University -- a world authority on Bayes's Theorem -- published an
entire series of papers alerting researchers to the "astonishing" tendency of
the standard statistical tests to mislead. 'Significant evidence'," Berger
warned, "can actually arise when the data provide very little or no evidence
in favour of an effect." The warning could not have been clearer. But again,
it was ignored.
In 1986, one scientist decided to take direct action against the failings of
Fisher's methods. Prof Kenneth Rothman, of the University of Massachusetts,
editor of the respected American Journal of Public Health, made a bold stand
and told all researchers wanting to publish in the journal that he would no
longer accept results based on P-values.
It was a simple move that had a dramatic effect: the teaching in America's
leading public health schools was transformed with statistics courses revised
to train students in alternatives to Fisher's formula. But two years later,
when Rothman stepped down from the editorship, his baon on P-values was
dropped, and researchers went straight back to their bad old ways.
The story in Britain has been similar. In 1995, The British Psychological
Society and its counterpart in America quietly set up a working party to
consider introducing a ban on P-values in its journals. The following year,
the working party was disbanded -- having made no decision. 'The view was that
it would cause too much upheaval for the journals.' said one senior
figure. All research submitted would have to be vetted by a panel of
Leading British medical journals have also considered taking decisive action
-- but they too have shied away. Instead, they are quietly trying to nudge
researchers towards alternative ways of stating findings. The most popular are
known as "confidence intervals" yet tbese too are known to suffer from similar
flaws to P-values. exaggerating both the size of implausible effects and their
Now. more than 30 years after the first warnings were sounded, it is clear
that the scientific community has no intention of taking decisive action to
tackle the critical flaws in Significance Testing. It has grown used to seeing
its supposed breakthroughs come and go: the flaky claims of health risks from
a host of implausible causes, the "wonder- drugs" that lose their amazing
abilities outside clinical trials.
Taking action would mean 'radical re-training". some scientists lamely
argue. Curiously for a profession supposedly dedicated to discovering
truths. the reliability of research findings is never mentioned. It is hard
to avoid the conclusion that the real explanation for all the endless evasion
is not scientific at all. It is simply that if scientists abandon significance
tests like P-values, many of their claims would be seen for what they really
are: meaningless aberrations on which taxpayers' money should never have been
The plain fact is that 70 years ago Ronald Fisher gave scientists a
mathematical machine for turning baloney into breakthroughs, and flukes into
funding. It is time to pull the plug.
Robert Matthews is Visiting Fellow in the Department of Computer Science,
Aston University, Birmingham. and Science Correspondent of the Sunday
Telegraph. A full account of the issues raised in this article. "Facts versus
Factions: the use and abuse of subjectivity in scientific research" is
available from the European Science and Environment Forum. 4 Church
Lane. Barton, Cambridge CB3 BE, price £3.
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Last modified 23 September 1998