The Burch-Fletcher correspondence
Two weeks after the confrontation at Hammersmith, Burch wrote to Charles Fletcher setting out his considered, critical judgement of Richard Peto’s lecture. They proceeded to discuss the draft of chapter 4 of the new RCP report by exchange of letter:
|Burch 29 June||Fletcher 6 July|
|Burch 19 July||Fletcher 30 July|
|Burch 5 August||Fletcher 15 September|
|Burch 29 September|
Fletcher’s letters are characterised by increasing impatience and incredulity that Burch does not find his arguments convincing. He simply cannot see that Burch has the superior intelligence and grasp of scientific method. In the final exchange, the two men put their cards on the table in a revealing discussion of their different outlooks: Fletcher the practical man with humanitarian motives and limited tolerance for what he regards as Burch’s sophisms and obsessiveness, Burch the visionary who wishes to make medicine a science to compare with physics and contemptuous of the soft science of epidemiology and its sloppy intellectual standards.
Fletcher on Burch:
When you came to lecture here, I got the impression that you were open to logical persuasion. As our correspondence has continued, I have become less and less confident of this and more convinced that you are the victim of an “idee fixe”. Another difficulty I find is that you seem to look on the whole question as if it were a problem in theoretical physics in which the outcome might be of practical importance in half a century from now. Doctors have to decide what to do about smoking and health now and the causative hypothesis is consistent with so many more and inconsistent with so many fewer facts than the pure genetic hypothesis that from the practical point of view, we have to persuade people not to smoke and to encourage the manufacturers to introduce less harmful forms of smoking.
Burch on Fletcher:
You are justified in contrasting our attitudes. Doctors do have to decide what advice they should give to their patients and I imagine that the majority would tend to err (if they err at all) on the side of caution. That is right and proper. But I am bound to say that humanitarian considerations, however noble, are almost certain to induce bias where the investigator is concerned. Those of us who have been trained in a more rigorous science than medicine ought to feel free (or even obliged) to challenge conclusions that appear to us to be unjustified. We frequently read claims in the medical literature that some environmental factor causes a certain disease. These claims often rest on the flimsiest of evidence and a wholly unjustified interpretation. Watch dogs are essential.
It is much more difficult to practice this method in medicine than in physics, and it is no coincidence that Popper is unpopular among certain epidemiologists.
There follow extracts from the correspondence quoting the two men in their own words as they consider the report section by section.
[The purpose of the reports]
Burch: Is it supposed to be an objective, rigorously argued. scholarly review that will withstand the scrutiny of a critical and well-informed scientist? Or is it intended primarily as anti-smoking propaganda with sufficient trappings of scholarship to influence the ‘average doctor’ and the ‘intelligent layman’?
Fletcher: The College Reports have all been aimed at “the intelligent laymen” such as a Member of Parliament or a reader of the “Observer”. We also hope that it will guide doctors without a specialist interest in the subject to see the general drift of the arguments.
4.1 [The increase in lung cancer]
Burch: The chapter begins tendentiously… it is not until we reach the second paragraph that we sense that the ‘facts’ might be suspect.
Fletcher: We have changed “was [a] rare form” to “appeared to be [a] rare form”.
4.2 [The association with smoking]
Burch: You refrain from commenting on the low mortality ratios found in Japan and India.
Fletcher: We have omitted a number of side-issues of minor relevance to the main topic.
4.5 [Correlation with smoking rates. Inhalation.]
Burch: I find it very strange that those heavy smokers who say they inhale have a lower incidence of lung cancer than those who say they don’t.
Fletcher: We know little about the way in which deep inhalation affects the site of deposition of tobacco particles. C. N. Davies… has considered that a reversal of this kind is what he would expect.
Burch: Do you have the reference? Would you advise heavy smokers to inhale?
Fletcher: A letter to the B.M.J. (1957, 2, 410). To advise heavy smokers to inhale would be like advising heavy drinkers to swallow.
Burch: I am impressed by his cautiousness… As I understand it, swallowing is an unavoidable part of drinking. But … some heavy smokers inhale and some don’t.
4.8-9a [Reduced rates of lung cancer after stopping smoking in different age groups]
Burch: Why has the age-range 20-64 been chosen in preference to 35-64yr used earlier?
Fletcher: Because that is what Richard Peto used in his analysis.
Burch: Why did Peto switch the age-ranges?
Fletcher: Because there were just a few doctors and one or two deaths in that age… It makes no difference to any conclusion.
Burch: Have you calculated the expected fall in death-rates, using the data for smoking habits?
Fletcher: It is difficult to relate changes in smoking habits to changes in mortality rates other than in the qualitative way in which we have done.
Burch: For those doctors aged 65yr and above, it appears that trends beyond 1967 were up rather than down.
Fletcher: We shall be making reference to these smoking habits when I have more detailed information.
Burch: Are diagnostic standards the same for doctors and the general male population?
Fletcher: [no answer]
4.13 [The constitutional hypothesis]
Burch: i) the habit causes the disease; and/or (ii), the disease causes the habit and/or (iii) one or more other factors cause, or predispose to, both the habit and the disease.
Fletcher: I don’t think that competent scientists should consider ridiculous hypotheses.
Burch: Competent scientists should and do consider ‘ridiculous hypotheses’ – if only to dismiss them.
Fletcher: Only if it is possible to disprove them.
Burch: I do not find the hypothesis [that pre-cancerous changes cause the smoking habit] to be ‘theoretically almost inconceivable.’
4.14 [“Sheer improbability” of the constitutional hypothesis. Adventists.]
Burch: This looks like the fallacy of omniscience… Seventh Day Adventists are not chosen randomly from the general population: appeals to common sense (or prejudice) have no role in a rigorous science.
Fletcher: Rigorous scientists who have no common sense are liable to miss the obvious… Heavily addicted cigarette smokers are unlikely to be attracted to an abstemious sect but this does not necessarily apply to all smokers.
Burch: No appeal to common sense will alter the fact that 7th Day Adventists are not chosen randomly from the general population.
Fletcher: I should perhaps explain what is implied by “an increased risk”. Residents in cities during the war had an increased risk of being killed by a bomb [compared] with residents in country areas but not all residents in urban areas were [killed] by bombs – the same applies to smokers.
Burch: We are now in agreement over the definition of ‘increased risk’. However, I think your analogy does not give a close parallel.
4.16 [Increase in lung cancer deaths in the twentieth century.]
Burch: As a general proposition the first sentence is false. Suppose, for example, that one or more of the genes that predispose to lung cancer associate positively with one or more of the genes that predispose to fatal infectious diseases. Then a diminution in the level of the fatal infectious disease(s) would be followed by an increase in the incidence of lung cancer.
Fletcher: Your hypothesis here demands much too much of the genes.
Burch: Are you confident you really know the scope and limitations of genes? What do you make of the association between cancer of the lung and tuberculosis…?
Fletcher: I rely on geneticists to tell me how unlikely the hypothesis is in this respect… In relation to the association between cancer of the lung and tuberculosis; the association is much too weak to explain the large increase on the part of lung cancer… I suggest you acquaint yourself with the “Berkson” fallacy
Burch: Are you confident that Professor Edwards… really knows the scope and limitation of genes?
References to Berkson are given in my book.
4.17 [Sex ratios discriminate real from artifactual change.]
Burch: We cannot be sure that the extent of underdiagnosis during the first half of the century and the extent of overdiagnosis during the past decade or so, were the same for both sexes.
You are correct in stating in the footnote that ‘This important statement is one which cannot be established by any scientific study’.
Fletcher: [I question] the highly selected hospital data which you quote in your book.
Fantasy is certainly a thing which you seem prone to indulge in.
Burch: [The data] can scarcely be described as ‘highly selected’.
I do not claim that all changes in the recorded incidence of lung cancer must be due to diagnostic error.
Fletcher: There was only a small difference between the frequency of autopsy and clinical diagnosis of lung cancer although there were considerable positive and negative errors. All hospital data are highly selected.
It is the sex differential that favours the former [the causal hypothesis].
Burch: ‘Highly selected’. This is not true of patients with lung cancer.
Bonser and Thomas found that ‘errors and untraced cases were proportionately higher in females’.
4.18 [Different sex ratios in different cohorts.]
Burch: [A couple of typing errors]
I’m glad you have no criticism of the interpretation of these figures.
Burch: Having undermined the basis of the interpretation of these figures by pointing to the hidden assumption I thought that further criticism was redundant.
4.19 [Recent changes in sex ratios.]
Burch: To judge from recent U.S. necropsy series, the popularity of the clinical diagnosis. ‘lung cancer’, especially as applied to the elderly patient, has changed markedly.
The report is silent about the decrease in recorded rates in women below the age of 45 years.
Only fully proved necropsy records are of any value.
Fletcher: We are not concerned in this paragraph with elderly patients.
Women have also been smoking cigarettes with considerably lower tar content.
We don’t have national rates for lung cancer based entirely on necropsy records
Burch: When the data are so unreliable we are not compelled to give an interpretation
4.20-1 [Effects of giving up.]
4.22 [Familial incidence: genetic factors are seen only in smokers.]
Burch: I am curious to know what interpretation you give of lung cancer in nonsmokers.
Fletcher: I don’t know how to interpret lung cancer in non-smokers.
4.25-6 [Other cancers.]
Burch: I agree that it is ‘difficult to interpret mortality trends over the past half century’ – especially in terms of the causal hypothesis – but it would be a grave dereliction not to make the attempt.
Fletcher: Our report is … not a detailed scientific appraisal of the causation of all cancers that are associated with smoking.
They don’t constitute a major public health problem.
Burch: I feel you are morally obliged to illustrate and deal with this recalcitrant evidence.
Fletcher: I am concerned with preventing unnecessary deaths.
I have no moral obligation to explain any phenomena which I don’t understand.
Burch: I find the data for cigarette-associated cancers other than lung cancer most intriguing because they have an important bearing on the more general problem of association and cause.