How does an agency (like Kelly Blue Book) approximate the value of a used car, given model, make and year?

I am looking for a heuristic methodology (such as looking at car insurance etc.) that Kelly Blue Book or similar agencies follow rather than a reference to a website which price used cars.|||every year when new dars dome out they are given a value, and this the value gets broken down into milage. condition. paperwork. up keep. and age after its taken off the lot (aka sold) there are actually two values red book which is what dealerships can charge for resale or give during a trade in, and then there is blue book which is a "private" sale value set for someone who wants to sell or buy a used car...|||Kelly Blue book, and the black book prices are determined by banks, and their minions, they set the price for used vehicles, based on condition, mileage, options, and how many were produced,


insurance companies have their own minions to come up with insurance rates, based on your drving record, cost of vehicle, how far you drive each year, coverage needed, and deductibles.|||local car dealers bodyshops used car dealers