If you actually read Steve McConnell's book the focus is really on accurate estimates more than data and rigorous planning. He makes it clear that a high level of precision is impossible in some cases. (Many people fail to appreciate the difference between accuracy and precision.)
I haven't read the book and was going to ask about accurate vs precise but found this helpful article discussing it as it relates to software here if anyone else is curious about that distinction:
http://itsadeliverything.com/accuracy-vs-precision-in-estima...
