You may be familiar with the Pareto principle, otherwise known as the 80/20 rule. When applied to project schedules, the first 80% of the project takes 80% of the budgeted time. Slightly less well known but still true, is the corollary: the remaining 20% of the project takes the other 80% of the budgeted time.
Have you seen this often when writing software, writing a book, painting a room, organizing your finances, or basically any other non-trivial project? Why does this happen? I think it is well-meaning hard-working people who are optimistic. People fail to be pessimistic enough about complexity and roadblocks. As a result, a large increase in effort is needed near the end of the project to keep it on schedule. Our optimism gets us in trouble.
So what do we do? Be pessimistic in your time estimate. Then double it. Seriously.
It’s better to finish earlier than expectations than later. As I keep telling students: with schedules, it’s more important to be accurate than optimistic. Also, take this quote from Star Trek III: The Search For Spock:
Kirk: “How long to re-fit?”
Scotty: “Eight weeks. But you don’t have eight weeks, so I’ll do it for you in two.”
Kirk: “Do you always multiply your repair estimates by a factor of four?”
Scotty: “How else to maintain my reputation as a miracle worker?”