If you refer to this line: "over 80% of maintenance effort is used for non-corrective actions." it doesn't say that 80% of the software development effort goes

If you refer to this line: "over 80% of maintenance effort is used for non-corrective actions." it doesn't say that 80% of the software development effort goes to maintenance at all... it says that, of the X% maintenance effort, 80% is not used for bugs. Then it gives an example for the accuracy of this number, 21% of bugs, so 21%+80% ~= 100% It doesn't attempt to explain what X is at all. It could be 10% or 90%, however in my experience, and according to other's point of view[1], 60-90% of time dedicated to writing code being maintenance time sounds legit note: modifying = 5 * writing new code (20%). Understanding is another matter, but also important: [1] http://blog.codinghorror.com/when-understanding-means-rewriting/ http://blog.codinghorror.com/when-understanding-means-rewrit...