The fact there are two principal guides – Glass’s and CAP – proves there are many indeterminate variables.
In a perfect world CAP and Glass’s prices would be identical.
In reality, the two experts have different analysis, different instincts and varying preferences.
The assumptions are different and the process of analysis is different.
Little wonder then that both experts get criticised routinely.
When in the same month one valuer can come up with £8,500 as a retail price for a car bought at £7,000, and the other can recommend £6,500 there is scope for pistols at dawn.
“We are in a constant state of tension,” said Rushmore.
In addition to spot prices, valuers have to estimate price trends. Tracking trends and giving narrative involves forecasting changing market taste, seasonality and weather.
Tension also occurs on the showroom floor as a result of the consumer magazines that attempt price guides.
The detailed price feeds are trade prices, while the publications, mostly monthly and focused on guiding consumers what they might be expected to pay, might be weeks out when they are finally scanned by the data-hungry consumer.
Mike Hind is spokesman for CAP research and publications. “Absolutely people still use CAP and Glass’s. Research is even more important now than it ever used to be.
"What is changing is the amount of information required by the readers.
"The more you do, the more people respond. The days when a monthly publication is sufficient have gone.”
The nature of the information medium is changing fast too.
The research has to be suitable for display on computers, iPads and mobile phones. The user base is growing, but the sale of hard-copy research is shrinking.
Customers are buying a licence and liking it. The trend is more and more licences, more customers and more digital data.
A 50-site dealer group is now one customer buying a licence for the whole operation rather than 50 or more staff getting a book each; more information, more change, more complexity.
Users of Auto Trader’s Dealer Portal Mobile app even get a profit indicator to understand the likely margin, price to pay and days to turn stock.
The changes and the complexities are good for the providers. More people are buying multiple information sets.
“Say you are asked to pay £400 for a CAP Black Book. If that prevents you paying £500 too much for a car just once you come out on top,” said Hind.