Just to copy a comment I made yesterday on the other thread...
"I don't know what the data set is, but it being based on number of incidents and repair costs I would hazard a guess that it's related to the costs of the warranty and how eager they are to do the business.
You first have to define reliability, and for me cost doesn't come into it. A single, expensive failure does not signal an unreliable car, but a number of small failures would. I want to know that the car will start, stop and get me to my destination safely - that's what I would call reliable.
IIRC from my 6 Sigma days, reliability is the assurance that components, equipment and systems function without failure for desired periods during their whole design life. Pushing cost to repair into the equation is wrong, and distorts the underlying facts."
The cost of repair has absolutely nothing to do with reliability, so WD are not being quite as straightforward as they could be (This is the definition from their website - "The UK Reliability index takes into account all factors of a repair, the cost of the parts and the frequency of failures").
They are quite disingenuous in what they say:
It's really quite simple. The higher the Reliability Index score, the worse the car is - the lower the score, the better.
As a guideline, the average RI number on the 250 models we compare is 100.The Reliability Index figure is calculated as a combination of:
the number of times a car fails,
the cost of repairing it,
the average amount of time it spends off the road due to repairs
the average age and mileage of the vehicles we have on our books.
Reliability is the first item - WD then muddy the waters by adding in cost, time, age and mileage variables. So complex cars come bottom of the list? That's hardly a huge surprise.
Other cars include:
BMW 7 Series - Poor, 232
Jag XJ - Poor, 201
So are these 50% more reliable than a CL or SL? You can't say. But comparing the SL to the Jag, the main difference is the cost of repair, not time off the road (although the average SL was also 20% older).
So does the increased cost of repair make the SL more unreliable?
BTW, I'm not defending one marque against the other, rather I'm questioning a disingenuous index (with no published method) that is labelled as "reliability".