For example, consider a service sold with one-month free trial. Before a customer cn use the service, he or she has to sign up for the trial. One obvious assumption, then, of the business model is that customers will sign up for a free trial once they have a certain amount of information about the service … Somewhere in the business model, probably buried in a single cell in a spreadsheet, it specifies the ‘percentage of customers who see the free trial offer who then sign up.’ … It really should be represented in giant letters in bold red font: WE ASSUME 10 PERCENT OF CUSTOMERS WILL SIGN UP. Most entrepreneurs approach a question like this by building the product and then checking to see how customers react to it. I consider this to be exactly backward because it can lead to a lot of waste.
Eric Ries, Lean Startup, (p. 96)