In a small software development project a single person can analyze requirements, perform design, generate code, and conduct tests. As the size of a project increases, more people must become involved — we can rarely afford the luxury of approaching a ten person-year effort with one person working for ten years!

There is a common myth that is still believed by many managers who are responsible for software development effort: “if we fall behind schedule, we can always add more programmers and catch up later in the project”.

Idem, ibidem (with adaptations).