Psychology 242
Introduction
to Research
Dr. McKirnan, Class Readings
March 15, 2002
Statistics misleading, some doctors say.
Relative numbers skew benefit to look much larger.
By John Crewdson
Tribune senior correspondent
Whether the breast cancer death benefit attributed to mammography is 20 percent, 30
percent or some other figure, those numbers represent the relative difference in the risk of
breast cancer death between large
groups of women who received regular
“Relative risk” – how much the likelihood of
mammograms for several years and
something such as contracting a disease
equally large groups who did not.
changes from one condition to another – is an
intuitively appealing measure. However, when
Even if the studies on which those
the overall rate of a disease is very low, a trivial
numbers are based, at least one of which increase in absolute risk can look very large
is being questioned, prove valid, the real
when expressed as a relative risk (that is,
benefit from mammography is much
doubling the risk of a very rare event still leaves
smaller than it may appear.
it rare, although a doubling of relative risk
sounds dire…).
Relative benefit "is OK for statistical
inferences," says Donald Berry, the chief
This is why it is important to really understand
of biostatistics at the M.D. Anderson
where statistical results come from. This article
Cancer Center in Houston, "but it is
illustrates the argument well in terms of the
meaningless for a woman making a
benefit of mammography in preventing breast
screening decision. Absolute risk
cancer, an area where the use of relative risk
reduction is what matters."
statistics has become controversial.
Numbers confuse even doctors
Relative benefit, which makes an absolute difference appear larger, is confusing even for
physicians, much less their patients.
When doctors in a study were told that a hypothetical medicine for high blood pressure
reduced a patient's chance of dying from heart disease by 20 percent--the relative reduction
in cardiac deaths--they were nearly twice as likely to say they would prescribe the drug as
doctors who were told that it lowered the death rate from 7.8 percent to 6.3 percent--the
same benefit in absolute terms.
Put another way, a 20 percent slice of a tiny pie is still a very small piece of pie. And breast
cancer, which annually kills about 40,000 out of some 63 million American women over 40,
is a small pie compared with major killers like lung cancer, heart disease and stroke.
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McKirnan, Psychology 242, Week 9 reading. Interpreting relative risks data
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The updated Swedish mammography trials to be reported Saturday in The Lancet, show a
current relative death benefit of 20 percent among women who have mammograms. That's
based on the determination that out of 129,750 women who were invited to begin having
mammograms in the late 1970s and early 1980s, 511 died of breast cancer over the next 15
years--a death rate of 0.4 percent.
In the comparison group of 117,260 women who were not invited, there were 584 breast
cancer deaths over the same period--a death rate of 0.5 percent. That is, indeed, a 20
percent relative benefit in favor of mammography. But the absolute difference between the
two groups after eight years of mammography is seven deaths a year in a female population
of 250,000.
Lars Werko, one of Sweden's best-known physicians, believes "it is useless to look at
mortality" when calculating the benefits of mammography. "It is much better to look at how
much the thing prolongs life," Werko says. "The question is, is it worth doing for such a short
prolongation of life?"
Werko notes that Finnish researchers attending a Swedish Cancer Society meeting last
month reported that that country's mammography screening program had so far added
between 10 and 12 days to the average woman's life.
"It may be difficult to justify a screening program . . . on the basis of the mortality reduction
only," the Finnish researchers concluded.
According to Berry, the latest Swedish estimate of a 20 percent relative benefit means that
the average woman in her 40s will gain a little less than three extra days of life. For women
in their 50s the gain is a little more than three days. For women in their 60s, it is eight extra
days.
Another way of measuring the benefit of mammography, or any other medical intervention,
is to calculate the number of procedures needed to save one life, an approach favored by
Dr. Russell Harris, an internist and clinical epidemiologist at the University of North Carolina
School of Medicine.
Harris says that when he takes time to explain the benefit of mammography to his female
patients in those terms, at least 1 in 10 chooses not to undergo the procedure.
"I don't go at it saying mammography is a bad thing," he says. "It's just that relative risk
reduction is not the way to go. It's necessary to understand the absolute risk. All I'm arguing
is that I want them to know that number."
Simplifying the decision
Rather than tossing around percentages and odds ratios, Harris asks his female patients to
imagine "a thousand people just like you. It's easier to understand that way."
According to Harris's calculations, if none of 1,000 50_year_old women ever has a
mammogram, 13 will die of breast cancer before they reach the age of 75--not a large
number to begin with because breast cancer, despite the attention it currently is receiving,
accounts for only about 2 percent of all deaths.
Next, imagine that each of the 1,000 women has a mammogram every year for the next 10
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McKirnan, Psychology 242, Week 9 reading. Interpreting relative risks data
3
years. Assuming the Swedish studies are valid, how will that affect the breast cancer death
toll?
Not by nearly as much as most women believe. Of the 13 women who would have died of
breast cancer without mammograms, 10 still will die of breast cancer.
The absolute benefit of mammography for women in their 50s, according to Harris, is three
lives saved--or, to be more precise, three breast cancer deaths avoided--for every thousand
women who have annual mammograms for 10 years, a total of 3,333 individual
mammograms to prevent one death.
If the 1,000 imaginary women in their 50s are replaced by 1,000 women in their 60s, the
number of breast cancer deaths avoided increases to six.
A less conservative estimate, by researchers at the University of California-San Francisco,
suggests between 2,700 and 5,400 individual mammograms are required to avoid one
death from breast cancer.
Ideally, saving even one life at any cost is a worthy goal. In the real world of finite healthcare resources, the costs, both economic and psychic, must be carefully considered.
"It is of course invidious to put a price on a woman's life," says British surgeon Dr. Michael
Baum, "but one has to ask, could these large sums of money be redirected both for service
and research in such a way that more women would benefit and fewer well women would be
harmed?"
But Dr. Steven Woolf, a professor of family practice at Virginia Commonwealth University,
notes that "the rest of us are interested in absolute benefits not because of monetary
concerns, but because of worries about the harms of interventions. The cost is a side issue
if the intervention causes more harm than good."
A recent study in The Lancet concluded that for every 10,000 mammograms given to
apparently healthy women, doctors will order an average of 647 "diagnostic" mammograms,
a more painstaking, and expensive, procedure to re-examine something that appeared
suspicious the first time around.
More tests follow procedure
Determining which women have cancer, The Lancet said, also will require 358 breast
examinations by ultrasound; 104 "aspiration" biopsies, in which a long needle is used to
remove fluid and cells from the area of the breast with the apparent abnormality; 209
surgical biopsies, in which part of the breast containing the abnormality is surgically
removed; and 500 additional doctor's office visits for physical examinations or surgical
consultations.
In the end, only about 25 of the 1,000 women will have breast cancer, because the great
majority of the "abnormal" mammograms will prove to be false positives. But the
mammograms and the procedures they spawn will have cost the health-care system about
$350,000 for each breast cancer death avoided, not counting the costs of treating those
women who actually do have cancer.
Copyright © 2002, Chicago Tribune
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