Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Further memorandum submitted by Professor Denis Mollison

TB IN CATTLE AND BADGERS: COMMENTS ON THE CHIEF SCIENTIFIC ADVISER'S REPORT AND ON THE ISG'S RESPONSE

  1.  This note follows up my earlier submission to the EFRA Committee for their session of 24 October, and includes responses to some issues raised in that session.

  I begin, at the request of their Chairman John Bourne, with a brief audit of the ISG's Response to Tuberculosis in Cattle and Badgers: a Report by the Chief Scientific Adviser.

AUDIT

  2.  Having considered carefully both the Chief Scientific Adviser's Report, and the ISG's Response, I am happy to endorse the ISG's main points, as set out in paragraphs ISG3-5, and their more detailed response as set out in subsequent paragraphs, including the key aspect of terms of reference (ISG10-11), and the six points concerning the interpretation of scientific data, statistical analyses, and modelling results (ISG12-27).

  3.  I trust it is not necessary to go into all the detailed arguments, since I have already set out my endorsement of the ISG's statistical and modelling work and its conclusions in my audit of the ISG's final report, published at the same time as that report in June 2007 (copy attached).[11] Among the conclusions of my audit (section 8 therein), I commended their conduct and analysis of the RBCT as a very thorough and sure-footed demonstration of good science aiding public policy in a complex and controversial situation.

THE CSA'S REPORT

  4.  The ISG's Response covers most of the six or seven specific criticisms to which I alluded briefly in my earlier comments submitted to the EFRA Committee. I will therefore comment here only on a few specific matters that are not so covered, as follows.

  5.  First, I stand by my general comment that the Chief Scientific Adviser's Report is inexpert and unbalanced.

  6.  I am sorry that the Chief Scientific Adviser has interpreted my accusation of lack of expertise as "attacks on personalities", which it was not. My point was that his report is mainly concerned with detailed discussion of complex statistical modelling and data analysis issues, yet his group lacked expertise in these areas. At the session of the EFRA Committee on 24 October, Sir David neatly confirmed my criticism in attempting to rebut it, when he set out the very considerable expertise of his panel in quite different areas (ecologist and badger expert; immunologist and microbiologist; epidemiologist; expert in veterinary medicine and animal clinical studies, specifically in the area of animal TB; developer of one of the critical tests for cattle TB). [see EFRA Committee Proceedings, Q384]

  It is worth noting in this context that the group that previously reviewed the ISG's work in 2004, chaired by Professor Godfray, did include an eminent applied statistician, Professor Robert Curnow.

  7.  Since the Godfray Report was referred to several times in Professor Woolhouse's evidence, including restating that report's downplaying of the purpose of the RBCT (see Q371) and it's likely usefulness (see Q380), I should like to make available to the committee my rebuttal of those points (please see Comments on Godfray et al, 2004, attached).[12]

  8.  As regards balance, the CSA's Report generally emphasises only points that downplay the detrimental effect of culling. A clear example of this one-sided view is described in the ISG's paragraph ISG15.

  9.  In my earlier comments I described the CSA group's epidemiological analysis as muddled in its discussion of the basic concept of R0. To explain: when as here we have disease in two interacting populations, here one=cattle, two=badgers, we have four parameters. For clarity, we should use four different symbols to refer to these—for example, R12 to denote the number of secondary cases in badgers caused by one infected cow—rather than using the same symbol R0 for all of them as in Annex 3. The overall R0 for the 2-population situation is a function of all four parameters (to be precise, .

From this little bit of mathematics it is easy to show that the statement in paragraph 11 of the CSA's report—"If R0 is below one for all the transmission routes, levels of infection will decline substantially"—is wrong; all four can be below 1, and yet R0 >1, so that the disease will persist. [For example, when each

  Rij = 0.6, R0 = 1.2.]

  The CSA's group's misunderstanding of R0 is presumably what led to their faulty interpretation of some of the ISG's modelling work (see ISG21).

  10.  A rather different point is that in this part of the discussion they implicitly include an assumption, which is more difficult to spot surrounded by mathematics, namely that badger removal will reduce at least the badger to badger transmission parameter (R22) and thus the incidence of the disease in badgers. Unfortunately, the evidence is that this assumption is untrue (see ISG25-26).

  11.  I should like to put on record my agreement with Sir David King's preference for avoiding appearing over-precise in the presentation of statistical results (Annex 1)—a point I have adhered to in my own reports on the ISG's work.

  12.  To finish with two general comments. First, the EFRA Committee were right to ask Sir David King why he did not discuss with the ISG his disagreements over their analysis and conclusions before publishing his report. One cannot help feeling that more light and less heat might have been generated had he done so.

  13.  Finally, the single most important difference between the ISG's and the CSA's conclusions stems from the latter's too narrow remit, as discussed in ISG10-11. One might rather have expected the CSA to have set a wider remit, so as to consider options for improved control of TB other than badger culling.

  The key point here is that a discussion of science that excludes the practical and economic aspects cannot be adequate as a basis for government action. The Chief Scientific Adviser's Report therefore cannot provide justification for it's strong and unqualified policy recommendations.

Professor Denis Mollison

Former Statistical Auditor to the RBCT

November 2007

REFERENCES

Mollison, D (2004) Comments on the Independent Scientific Review of the Randomised Badger Culling Trial and Associated Epidemiological Research by Godfray et al, http://www.defra.gov.uk/animalh/tb/isg/pdf/audit.pdf

Mollison, D (2007) Statistical aspects of the Randomised Badger Culling Trials: Final Report, June 2007, www.defra.gov.uk/animalh/tb/pdf/statsaudit rpt0607.pdf







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