Select Committee on Agriculture Appendices to the Minutes of Evidence


Memorandum submitted by Dr Fiona Mathews, University of Oxford (J12)

  I would like to submit the enclosed paper for consideration by the Agriculture Select Committee. This raised serious concerns about the statistical power of the badger cull. I was alerted to these issues some time ago, when I discussed the design of my own project (an investigation of the Risk to Cattle from TB in wildlife other than badgers (SE3009), MAFF, £960,051) with members of the Independent Scientific Group (ISG). My figures show that it is probable that the trial, as it currently stands, has statistical power of only 50 to 60 per cent to detect a reduction in TB incidence of 20 per cent within five years, not the 90 per cent power assumed.

  It is appropriate to draw to the committee's attention the fact that the enclosed paper has not yet been published in the scientific press. It was recently submitted to a leading academic journal. Although five of the six referees agreed that the points raised were fundamentally correct, the editor concluded that I would find myself "considerably outgunned" in what might be very public fora. In addition, several referees raised concerns about the use of simulated rather than real data on breakdowns in the trial areas to illustrate the points made. The summarised data published to date are unsuitable for sample size calculations.

  I therefore respectfully request that the committee asks:

  1.  What is the minimum reduction in TB incidence that the ISG consider important to detect, in both proactive and reactive areas, and within what timescale? (The Krebs Report cites a 20 per cent reduction within five years).

  2.  What reduction in TB incidence do the ISG expect to detect, given the data available to them, in both reactive and proactive areas?

  3.  What do the ISG consider is the power of the trial to detect such reductions?

  4.  Why are no trial areas located in Wales? The trial zones are meant to be a representative sample of all areas with high rates of repeat and contiguous breakdowns in order that the results can be generalised. Two areas of Wales fall into this category but are not being sampled. The Policy Unit of MAFF have denied anecdotal reports that the Countryside Commission for Wales declined to approve the trial.

  5.  Will MAFF now allow historical data on the incidence of TB in the trial areas (prior to the commencement of the current trial) to be made publicly available? It is important that such an anonymised database permits the identification of repeat and contiguous breakdowns.

  6.  Will MAFF commission the collection and publication, by independent observers, of data on the survival of badgers within trial areas (including sites where permission for culling was denied)? Considerable badger survival is widely reported by local Badger Groups. This contrasts with information released by MAFF. Neither source of data has been independently confirmed. Badger survival clearly has important implications for the extent to which TB incidence in cattle can be expected to fall.

  7.  Will the Veterinary Laboratories Agency release historical data on the proportion of badgers, culled in previous badger removals, found to have open lesions at post mortem? These data, indicating the prevalence of infectious individuals, are important to build better models of the epidemiology of TB in badgers and cattle, and assist in understanding the likely impact of the cull.

  8.  What, explicitly for both proactive and reactive areas, is the mechanism being tested in the badger cull?

  If the aim is to reduce TB in cattle by removing all (or most) infected badgers in particular areas, then the completeness of the cull is an important issue. Further, it is difficult to see how a "reactive" cull could be effective, unless the trial is primarily concerned with repeated breakdowns at the same or adjacent site. It is important in this case to note, as discussed in the attached manuscript, that repeated and contiguous breakdowns are not independent of each other and pose particular difficulties for sample size calculations and analysis.

  Conversely, if the aim is to reduce badger density generally in trial areas to below a threshold level, such that transmission within the badger population is reduced and TB maintained at a low prevalence, then there are important policy implications. Badger density would need to be maintained at low levels for long periods. This scenario contrasts with the information I understand was presented to the Bern Convention by the Central Science Laboratory, which suggested that the culls would not depress badger density in the longer term. Finally, unless there are a great many culls within the reactive area, the potential for lowering badger density is, in any case, limited.

27 October 2000

previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries index

© Parliamentary copyright 2001
Prepared 10 January 2001