As we build ever larger and more powerful systems it becomes ever more important that those systems, and the components of which they are made, should be transparently simple and self-evidently correct. As Professor Dijkstra points out (Structured Programming, Academic Press 1972):

If the chance of correctness of an individual

component equals *p*, the chance of correctness

of a whole program, composed of *N* such

components, is something like

*P = p ^{N}*

As *N* will be very large, *p* should be very, very

close to 1 if we desire *P* to differ significantly

from zero!

The purpose of this book is to present a coherent method and procedure for designing systems, programs and components which are transparently simple and self-evidently correct.

Principles of Program Design

by M.A. Jackson

Example: Suppose the chance of correctness of an individual component is 99% (p=0.99) and the program is composed of 500 such components (N=500). The chance of correctness of the program is

P=(0.99)^{500 }P = 0.007

There is less than a 1% chance that the program is correct!