Time spent on requirement and its effect on project success and development time
See Code Complete, By Steve McConnell, Table 3-1.
He compares the average cost of fixing defects based on when they're introduced and detected.
Detection at construction time costs 5-10 times more than detection at requirements time, and 10-100 times more post-release.

The table is based on the following sources:


"Design and Code Inspections to Reduce Errors in ProgramnDevelopment" (Fagan 1976)
Software Defect Removal (Dunn 1984)
"Software Process Improvement at Hughes Aircraft" (Humphrey, Snyder,
and WIllis 1991)


and several more