Garbage in, garbage out
The title tells you one of really important feature of any software or a model. (I’m a bit irritated by some comments concerning the economic crisis and whose fault it was as some people are blaming the models that clearly state that they don’t work in certain conditions…).
Let’s demonstrate this: you want me to do a painting about the blue sky with red paint only. Okay, some might say that this is great artistic challenge but what it really means, is that we will end up with a red painting that has no blue in it. No matter how hard you look at the painting, it still will be red (unless you magically can look it via some strange lens or something).
This is what was wanted:
This is what we got:
Notice: The images are copies except for the colour. There are lots of features and probably the things we wanted which may cause us to believe that the result is correct especially if we would be talking about numbers and not about paiting's main colour.
Your program can’t function properly if all its inputs are twisted and aren’t helping the program to reach the wanted result. You can’t say to a program to calculate 1+1 if you actually want the result of 2+2. Mostly the problem of course isn’t this simple because we are calculating or analyzing something much more complex. Computer will calculate almost anything for you, you just need to know if you are asking the right thing. Computer doesn’t really know what it is doing; it just does what it is told. So, we still need humans to see if something makes any sense. :)
Another point: a model is just a model. It isn’t a fact. It is something we have noticed that might describe some situation with some accuracy AND often with certain prerequisites (look at the original Black-Scholes’ assumptions list). It may not work all the times and it definitely is likely to be unfunctional with a totally new twist of events if the model isn’t very fundamental and well done.
A model isn’t a fact. It is called a model for a reason and therefore it does have its limitations.
PS. This “garbage in, garbage out” has been invented before modern computers by Charles Babbage!