This is a summary of my more detailed post on Friday, outlining the key points.
- The DfT surveyed traffic in June and July.
- They waited until September to ensure no late corrections to the data.
- By this time many of the bikers surveyed had taken their bikes off the road for the winter.
- At this point the DfT checked the figures against the DVLA’s VED database.
- The bikes which had been removed from the road were mistakenly assumed to be evading tax.
- The error was then amplified by a “corrective” assumption that tax dodgers would use their bikes less and get missed by the survey, the number of evaders would be underestimated. This one step doubled the number of bikers assumed to be riding without tax and it did this because it assumed bike mileage figures would match evasion figures in the same way as they do for trucks (The only category of vehicle they have stats for). This doesn’t take account of the important fact that the average motorcycle covers many, many fewer miles than the average trucks. That means that the people you see most often are not necessarily people traveling furthest as they would be for trucks, but are much more likely to be people who just live nearby to a survey site. This means that the assumption that you will see lower than actual levels of evasion (because tax dodgers travel less far) is undermined, as only a very few of the people on bikes are travelling large distances.
- Because of the small number of motorcyclists surveyed, the DfT’s own figures show that the margin for error would be at least 20% either way even if the incorrect assumptions were to have been true.
What does this mean?
These are quite major flaws in the methodology of the survey and (I think) blow apart the reported figures. The headline figure was extrapolated from an “observed” figure of only 16% on the basis more tax evading motorcyclists would have been missed, as they don’t travel as far, which I’ve shown above is almost certainly a flawed assumption. If only around 10% of the riders had SORNed their bike at the end of August or during early September, the vast majority of the “untaxed” bikers would disappear from the stats. Add to that a 20% margin of error, because of the small survey size and the figures may well be comparable with the rates for cars. A precise figure is going to be very difficult to arrive at, as no-one currently has the relevant data that could quantify the errors more precisely.
Whose fault is it?
The mathematics used in the statistical modelling was all applied correctly. The errors arose because of mistaken assumptions about how motorbikes are used and would probably have been spotted if a single representative of the motorcycling community had been consulted at the design stage of the survey. What probably should have been spotted is the ridiculously high figure of 38% evasion, which should, I believe, have raised alarm bells. I suspect that this is why Southampton University were asked to double check the result, but they only checked the statistical techniques used, and did not carry out an assessment of way the VED data had been obtained nor of the validity of the underlying assumptions.
So the blame for all of this lies with whoever designed the survey and data processing methodology, and not with anyone who actually carried it out.
I think at the very least all bikers are owed an apology from Edward Leigh MP, of the Public accounts committee for his intemperate remarks. And another apology is due, I feel, from the DfT, for managing to balls up the figures in quite such a spectacular fashion.