The FICO simulator, being a product of Fair Isaac, presumably knows the complete FICO algorithms.
I would also presume that the simulator could, if designed that way, return an exact number if isolated to one change.
There is a very good reason why the simulator does not do that. It is called a trade secret.
When anyone develops a new product, they can choose to protect it one of two ways... they can either patent it, or they can maintain it as a trade secret.
Going the patent route may be the norm, but is not the best way to protect all inventions. To obtain a patent, one must present a full written disclosure of how to make and use the invention. It enable anyone, as soon as the patent is published, to copy and use it. In exchange for that public disclosure, the inventor gets an exclusive right to exclude all others from making and using the invention for a period of 20 years from the date they applied for the patent. So if anyone copies and uses the invention in any commercial way, the patentee can sue them for patent infringement while the patent is still in effect. When a patent expires, it is non-renewable, so the inventor gets 20 years, then all protection is gone.
There are some inventions, such as Coca-Cola and the FICO scoring algorithm, that by their nature cannot be copied by just having the product in your hand, and tinkering in the lab to "reverse engineer" it. Parties who have such products can gamble and choose not to make its innards public, and choose to protect the invention by maintaining it as a trade secret rather than publicly displaying its workings by obtaining a patent. When a party chooses that route, their decisions is irreversible, and their trade secret could live forever provided no one is able to reverse engineer it. So they must zealously protect it from public disclosure. One is permitted to reverse engineer a product provided they dont illegally obtain information from the owner of the product, and this protection is assured the owner by way of confidentiality agreements signed by its employees.
If Fair Isaac were to make public a detailed simulator that permitted one to input a single parameter and get a precise output value, it would vastly improve the capability for one to reverse engineer at least that part of the FICO scoring method. Given enough inputs and outputs, one could probably reverse engineer most of the algorithm.
They are not going to hand out their trade secret in such a fashion. So the simulator intentionally does not permit one to isolate single factors and get a precise output. Rather, it sums up scoring against all other categories, and usually only permits one to see an output that is months down the road, and not instantaneous.
It makes unknown assumptions based on the complete file, and calculates the results based not only on the one isolated factor, but many factors.
You wont see a precise FICO score simulator any more than you will see a publication in the NY Times tomorrow of the detailed code for their algorithms. You get fairly broad ranges over different periods of time. Clearly better than no "simulation," but not what most consumers want.
Starting Score: 675
