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


Examination of Witnesses (Questions 429 - 439)

WEDNESDAY 24 OCTOBER 2007

PROFESSOR JOHN BOURNE CBE, PROFESSOR PROFESSOR CHRISTL DONNELLY AND PROFESSOR ROSIE WOODROFFE

  Q429  Chairman: I do not know who wants to start but it might be quite interesting to take the views of the former ISG members about what has been said. This may not be peer review in the accepted sense but it is the best that we can manage.

  Professor Bourne: Would it be helpful if I put some structure onto what has gone before because I can understand why you are confused?

  Q430  Chairman: Yes. Structure is always helpful.

  Professor Bourne: First of all let me make the point that the comment was made that we did an experiment. That is not true. What we did was carry out a trial of future policy options, one of which was reactive culling, the other was proactive culling, with an expectancy at the time that these would be rolled into the field. Government subsequently determined that this was not going to happen. They eliminated the Wildlife Unit and if there was going to be any culling in future farmers had to do it. However, this was not just an experiment. The whole trial was designed by us to provide a range of epidemiological evidence and, my goodness, it is a good job we did it that way because that epidemiological evidence is so critically important. That is point number one. Point number two is that it is suggested that the evidence is confusing. I do not think it is confusing at all. I think we have shown very clearly that localised culling makes the situation worse and we provide the very plausible hypothesis why that happens, namely, perturbation, but since reactive culling has finished the negative effect of reactive culling has gone. I think that is quite an interesting observation. If you stop culling, stop perturbation; the negative effect of reactive culling goes. We have shown that reactive culling as carried out makes the situation worse, and we associate that with greater perturbation, but all the issues around that lead to an increased incidence in cattle and we are suggesting there is a link between the two. With proactive culling, which in our hands led to something like an 80% culling efficiency which resulted at the end of the day in something like 70 to 75% removal of badgers, and a large proportion of those that were left came in from the outside; they were immigrant badgers, we did find overall in 100 square kilometres, the area mentioned in the King report, something like a 23% reduction. There is no question that badgers contribute to cattle TB. In the very centre of the area it was higher than that. The modelling suggested overall that the impact badgers were making on the cattle TB problem in the areas in which we were working was about 30%, 30%, 40%, it was about that. The rest was down to cattle. We put forward ways in which we believed the cattle element could be tackled, and Sir David has accepted all of that; not in his report but certainly in media reports he accepted all of that. The issue comes down to how do you deal with the badger component. There is no doubt that culling has an impact. The problem is that although you have a positive impact in culled areas it is this perturbation issue that creates a problem. This, I think, is where it would be useful to spend a few moments discussing this. I am aware that in addition to the King report Defra sought in-house reports from the Central Science Laboratory (CSL), the Central Veterinary Laboratory (CVL) and Natural England. I have not seen these but I know they were commissioned, and within Natural England and CSL there are high-class ecologists, but I am sure you would find these reports of value too. I suggest you ask for these and study them; they do provide you with further information which I believe could be helpful to you. What we have shown is that perturbation is an issue. I am bound to say that when we started this work I certainly did not believe that perturbation was a problem. We knew about it but I did not think it would be an issue, but the data we have suggest it is an issue. In the King report I think they have underplayed perturbation. I will ask Rosie to talk about this in a moment. It would appear that they have not read all the literature. There are a number of papers that have been published on this which they do not appear to have read and I think they are overestimating the impact of geographical boundaries. They are making a number of statements on the value of one kilometre of arable land, for which there is absolutely no data whatsoever, and they are really saying, "There is a problem there but we can overcome it in some way by believing that we can". With respect to the size of the areas, we did extrapolate to 300 square kilometres or more. Sir David said we found significant impact of badger culling by that extrapolation. That is not true. We reported a modest difference between the positivity and the negativity. We then questioned the economics of the thing. You asked me earlier about comments that Lord Rooker had made and I think he criticised us with respect to the economic analysis. If you look at our first report, when it was suggested that a cost analysis be done, it was Lord Rooker who asked us to put an economist on the committee, so it was quite clear that we were going to provide some economic information, which we did. We did not do a cost benefit analysis; there were no benefits, and it is fully explained in the report. The other thing we found was that if one were to get the same effect that we did culling had to be done over the piece simultaneously, it had to be done in a co-ordinated manner and it had to be carried out for a very long period of time. In other words, it was not a trivial thing to do. We discussed this at length with respect to licensed culling that Defra firmly put on the agenda in our final report, and our advice was that it was totally inappropriate to expect farmers to do this. It could only be done using expert fieldsmen who would require an immense organisation and logistical activity to do it. Perhaps Rosie could pick it up from there and discuss the perturbation issue.

  Professor Woodroffe: Certainly.

  Q431  Chairman: Just before you do, and I am sorry if this is going to be a little bit tedious but it is very central to the question of this perturbation effect, going back to what Sir David's report said, in paragraph 27 it says, "The results are hard to interpret but it would appear that there is some evidence for an increase in cattle TB outside the removal area". They then go on to discuss the two-kilometre zone and its statistical impact, and they then say, "The increase was not statistically significant", so that is an interpretive point on the data that you had, which I would be grateful for your comment on. They then go on to make a third point, that there was an apparent small beneficial effect in the area up to half a kilometre from the removal area, so they are saying that things get better just across the boundary, which is an interesting observation. Then later on, after some more commentary, they say that perhaps perturbation—and I hope I have not misunderstood paragraph 31—"indicates that the detrimental effect seen outside the removal area may well be transient". In other words, you saw something happening but it goes away after a while. I guess that must refer to their subsequent observations about the comings and goings of badgers, but then they say in paragraph 37, "However, there is a reasonable possibility that the disruption is transient. The data do not discount this theory. If the disruption is transient the single regression model for the effect of perturbation used by the ISG is not adequate", so that is, if you like, an attack on your methodology. Perhaps you could, bearing in mind we are not statisticians and not experts, help us to address that critique of your information.

  Professor Woodroffe: I think you have absolutely put your finger on a very important point. Whilst Sir David King and Professor Woolhouse have repeatedly reiterated that they think there is almost no disagreement between the ISG and themselves, you have put your finger on the key area of disagreement, and I think that the reason you were seeing shaking heads behind them is that this particular issue of detrimental effects which they considered unconvincing and hard to interpret we considered very strong. Perhaps Professor Donnelly would be best placed to address the statistical issues and then I can come back to the ecological issues.

  Professor Donnelly: As a statistician I feel I should declare one interest, which is that I am going to use "significant" in its statistical sense, which is that if it is a significant effect the 95% confidence interval does not include zero. I think that is a bit different from the way John was just referring to it when he was saying, "David King said it was significant, I say it is modest".

  Q432  Chairman: You talk about margins for errors. We watch opinion polls. Those are the crude numbers we understand.

  Professor Donnelly: There are a few things that are going on. One is this issue of between culls one and two, and so let us address that first, and then we need to talk about what we are using to test the detrimental effect. Between culls one and two the breakdowns that will be detected will include breakdowns that resulted from infection events that happened before the first cull and infection events that happened between those two culls, because testing was every year and so it includes over that one-year time period. Keep in mind that we are always comparing like with like in terms of proactive areas with survey-only areas, and so we are comparing those same time periods. What will happen is that in that first year there will be a bias in the estimate towards no effect because some of what we are observing will be from the no effect period before any culling took place. Therefore, rather than there being any invalid period when we might get something spurious, we are actually getting an estimate that is biased towards the null because some of the differences will not be detected until the period between the second and third culls.

  Professor Woodroffe: So it is conservative.

  Professor Donnelly: Yes. We did not place any more emphasis on that particular usually year period than any other year period, but we considered them all in sequence. When we did that we found a non-significant trend towards a decreasing detrimental effect outside. We did address that. We discussed it both in the report and also in the IJID paper which David King cited. However, one of the things that we want to consider is that when doing sub-group analyses you have less precision, and I draw people's attention, if you have the final report, to page 99, which shows estimates stratified by which culls they were between, and I have the IJID paper which has the same graph as figure 2 right in the middle, so I will distribute it. It is either page 99 of the final ISG report or figure 2, which is right in the middle of the paper I am handing round. These figures show the variation in our estimated effects—

  Q433  Mr Drew: Are you looking at table 2 or figure 2?

  Professor Donnelly: Figure 2.

  Q434  Chairman: Figure 2, page 304, top right hand corner.[6] Just take us through the construction of the picture that we are going to see before you explain it to us because some of my colleagues—and this is not attacking their ability to understand—may not quite be keeping up, and the audience behind us are not where we are, so you will just have to bear with us whilst we have this explained to us, and if anybody does not understand perhaps they would indicate and ask as we go along. We will do our best to understand.

  Professor Donnelly: The top figure shows divided-up estimates for inside the targeted culling area and outside the culling area, and so the estimates are divided up in the top figure by cull sequence between culls one and two, two and three, three and four and so on; four and more get lumped together. What you see is that consistently in all four of these time periods outside trial areas in this two-kilometre zone there is a negative effect of culling which is detrimental above the line, and below the line it is a beneficial effect of culling. The beneficial effect of culling appears, based on the point estimates, to be greater after more culls. I wanted to point this out because there was this point that a simple regression is not an adequate way of looking at this. We very specifically presented this data so that people could see exactly what the estimates were. A regression will assume, say, that there is a linear effect, so you can put a straight line through. Here we are not assuming that. There is a "connect-the-dots" between them so that you can connect them up, but there is no assumption of linearity there; it is purely the data, so you can see wobbles and variations cull to cull. In each of these there is a non-significant but general trend towards a decreasing detrimental effect with more culls outside and increasing benefits, so both things move towards a better situation after more culls.

  Q435  Mr Gray: So what you are saying is, the more badgers you kill the less TB there is?

  Professor Donnelly: After you have culled for a longer time.

  Mr Gray: All this thing goes all the way across this and I cannot understand what you are talking about.

  Chairman: Do not attack the messenger. She is doing her best to explain it in her language to us as lay people.

  Mr Gray: I do not think so.

  Chairman: You have drawn a conclusion now, so you want to test out whether you have got the right message.

  Q436  Mr Gray: I am trying to understand what she is saying. Would you mind expressing it in language that a thick fellow like me can understand? I am sorry about that.

  Professor Donnelly: More annual culls, better results.

  Chairman: Now you have got it. He is in line for a degree already.

  Q437  Lynne Jones: You said non-significant.

  Professor Donnelly: I was talking about the point estimates. We looked at what the net effects were, and, balancing these two different areas out, pluses and minuses, the net effect between the first and second culls is consistently detrimental overall, looking at those two counteracting forces, but in all the ways that we looked at it with different sorts of linear effects in different databases we get a net beneficial effect after four culls. That still means that that is a net beneficial effect judged over the whole area, and of course that still involves some areas experiencing a detrimental effect and some areas experience a beneficial effect. The next thing I want to point out is on the graph below that, where this is all time periods taken together but we have now sub-divided by how far from the boundary, and this is probably more difficult to see. Think of the line that goes vertically in the middle of the graph as the boundary between the targeted area for culling and the untargeted area. As you move to the right you are getting deeper and deeper into a trial area and you will see that the estimates move towards a more beneficial effect. That was a non-significant trend but you do see quite a lot of consistency there. That is where David King was quoting about 40% benefit by the time you get deeply inside these trial areas. Outside the area targeted for culling there are four estimates dividing up the zone up to two kilometres out. You see an estimate of beneficial and then three detrimental effects. In that first zone between nought to 0.5 kilometres outside the culling area some of that area was actually culled, and the reason for that was that the trial area boundaries were drawn up and then after that areas were targeted for culling based on Rosie looking at maps and approving boundaries so that whole social group territories were targeted, and Rosie can talk about why that is. Some of that area between nought to 0.5 actually received culling and it is quite plausible that that is why there was a beneficial effect there, but beyond that you see three detrimental effects.

  Q438  Chairman: Whoa, whoa. In the report which Sir David King did he talked about, if you like, doing some work in the margin, in the area which would be the perturbed area. Is that what you are effectively saying you subsequently did, which was to go back and when you have got the extent of the social groups of the badgers do a bit more culling but outside the boundary?

  Professor Donnelly: It was not going back. It was done from the outset.

  Professor Woodroffe: What we did was, we started out with trial areas which we defined on the basis of property boundaries. The entire area plus land up to two kilometres—usually not as far as two kilometres but a maximum of two kilometres—outside was then surveyed for badgers. For all of the areas before they were allocated to treatments they were surveyed in that way, and then we went and inferred from the field data where we thought the boundaries of the social group territories fell, and we did that because we were concerned about perturbation and we thought we could minimise perturbation by trying to remove complete social groups, so we wanted to make sure that every property that fell inside the trial area was going to get the full culling treatment. We did not want incomplete culling on the outer area occurring because perhaps the main setts of the badgers that lived on this farm were just outside.

  Q439  Chairman: So basically you refined the cull areas to be as complete as possible in terms of the social areas for the badger setts you had identified, and therefore in any commentary which subsequently comes about, if you like, the statistics of perturbation take that refinement of the boundaries into account?

  Professor Woodroffe: Yes. The culling areas were slightly bigger than the trial areas and that is made very clear in all sections of the report. That is probably why you see that potentially beneficial effect just outside the trial area boundary, because a lot of those farms were receiving culling even though they were not inside the trial area.

  Professor Donnelly: Could I make one more comment to discuss these graphs? The boundary you see on each one of these points, to give you an indication of uncertainty, are 95% confidence intervals, but we had a concern in terms of the interpretation of these 95% confidence intervals in the David King report looking and saying, "Okay, outside trial areas three of the four include zero; therefore this does not look like such a secure result". If you look on the right hand side, all five of those include zero and that does not mean that the beneficial effect inside is insecure. Those confidence intervals are in a sense your level of uncertainty if you are just looking at one of these points. What we were doing here was showing how well each one of those points was estimated individually to justify where we thought we could get benefit by fitting a linear regression, and that is what we did for the cull number effect for the figure above. For the figure below we fit a linear effect looking at distance from the border inside trial areas. Outside trial areas there was no evidence, looking at this, of any linear trend.


6   Donnelly, C et al (2007). "Impacts of widespread badger culling on cattle tuberculosis: concluding analyses from a large-scale field trial", International Journal of Infectious Diseases, Volume 11, Issue 4, July 2007, p 300-308. Back


 
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