Treat a startup as an optimization problem in which performance is measured by number of users. As anyone who has tried to optimize software knows, the key is measurement. When you try to guess where your program is slow, and what would make it faster, you almost always guess wrong.

Number of users may not be the perfect test, but it will be very close. It’s what acquirers care about. It’s what revenues depend on. It’s what makes competitors unhappy. It’s what impresses reporters, and potential new users. Certainly it’s a better test than your a priori notions of what problems are important to solve, no matter how technically adept you are.

— Hackers & Painters: Big Ideas from the Computer Age by Paul Graham