Productividad en modificaciones al software

Yo siempre había pensado que era mas fácil y rápido hacer nuevos proyectos de software que modificar los antiguos pero leyendo PSP: A Self-improvement Process for Software Engineers me encontré con un par de citas con las cuales avalar mis impresiones.

"the productivity for software modification is often much slower than for new development"  Flaherty, M. J. "Programming Process Productivity Measurement System for System/370." IBM Systems Journal 24, no. 2 (1985): 168–175.

"an unpublished IBM study found that small code modifications were 39 times as error-prone as new development when measured in defects per modified LOC" - Managing the Software Process by Watts S. Humphrey