Simplicity is the ultimate sophistication.
Leonardo da Vinci
I have no data yet. It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
Said he.
If I had only one hour to save the world, I would spend fifty-five minutes defining the problem, and only five minutes finding the solution.
Albert Einstein
You fit a model, do the best you can, be open about your methods, then invite criticism. You can then take account of the criticisms, include more information, and do better.
Said by a fine modeller - A. Gelman