Why do your MOT charts show above average for the Skoda Superb?

I think you need to look closely at your "MoT results" statistics. I've just looked at the Skoda Superb. The graphs state that for every year apart from 2002 the Superb has a worse pass rate than the average. But when I look at the bar chart that evaluates the same data based on mileage the Superb is better than the average for every mileage. These stats can't both be true, can they? And, if these are wrong, how accurate are they for every other car?

Asked on 30 March 2013 by TW, via email

Answered by Honest John
Our mathematical genius explains:

“Both stats can be true, though it's unusual to see such a marked difference between the graphs. If Superbs cover rather higher than average mileage then it's possible for an average Superb of any age to have a worse pass rate than an average car of its age (which will have covered lower mileage on average) but a better pass rate than an average car of the same mileage (which will be older on average).”
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