What Car? guide to car supermarkets - How we scored each supermarket

CHOICE OF CARS (10 points max)
The larger the stock, the more likely a supermarket is to have the car you want. Bonus points were awarded for sites offering an especially wide mix of makes and types of car, while extra points were available if we managed to find cars on site that we had seen recently on the company's website.

PRICES (25 points max)
We took a sample of cars from each site and compared prices with those in the trade guides. We calculated the average saving from the sample; extra points were awarded to those outlets with particularly competitive savings.

HIDDEN CHARGES (5 points max)
Supermarkets which didn't have sneaky administration fees or hidden charges scored five points; those that did were deducted points.

FINANCE (5 points max)
Supermarkets offering finance scored one point. Bonus points were available to those offering a range of packages (hire purchase and PCP, for example), competitive APRs or 0% finance.

WARRANTY (10 points max)
Full marks given if the supermarket offers a one-year warranty. We then operated a sliding scale to apportion points for outlets offering six months' cover, three months' and one month's.

BREAKDOWN COVER (5 points max)
Outlets offering breakdown packages as standard scored points here – with more points available for longer cover.

TEST DRIVES (15 points max)
If test drives were available on all cars, the supermarket scored top points. Marks were deducted for restrictions on test-drive availability.

CONDITION OF CARS (5 points max)
Points were awarded for the quality of cars on offer on our visit. The better the condition, the higher the score.

PART-EXCHANGE (10 points max)
Our investigators took the same car and asked for a part-exchange offer. According to the trade guides, our Ford Mondeo 2.0 TDCi (130bhp) Zetec on an 03-plate with 25,000 miles was worth £6500 when we visited. We awarded the most points to the outlets that gave us the best offer.

MYSTERY SHOP (10 points max)
A supermarket can tick all the right boxes in a survey, but they won't be a good place to buy a car if the service is poor. Our experts visited every outlet, and this is their personal view, with their impressions of the quality of service and friendliness and helpfulness of the staff.

Those which made our team feel welcome and which offered a good deal won the highest scores. Those which put the pressure on or used wily sales tactics lost points. The subjective element of our survey accounted for just 10% of the available marks.