HP-UX Floating-Point Guide

Chapter 3 95
Factors that Affect the Results of Floating-Point Computations
Floating-Point Coding Practices that Affect Application Results
Another technique, which does not require you to use additional
functions or even to modify your code, is to make very small changes in
the input data and to observe the amount by which the result changes.
Wild swings in output magnitude may indicate an ill-conditioned
application.
Fixing an ill-conditioned application requires a thorough understanding
of the computations executed by the application. Chances are that the
instability is caused by very few intermediate computations—possibly by
just one. You must try to identify the location of the instability using
your knowledge of the application, of the characteristics of the math
functions called, and of the data being processed.