It is said that increase in the appraisal costs (i.e. costs spent for finding non-corfomances - testing, reviews) decreases the failure costs (that comprise of internal (fixing of bugs we found + retesting ) and external failure costs (fixing bugs that customer found + retesting).
Well, I think it is only true if the cost for defects found by customer is higher. What I mean is:
when I invest in the testing, the cost of rework of defects it discovers will be more or less the same as if the customer discovered it. My developers are paid the same, no matter whether they fix a bug found by the customer or ourselves. Of course, there will be some overhead included and I do not take into account indirect costs like our reputation going down for producing buggy software.
But I just do not believe the failure costs decrease as appraisal costs increase, as several books state.
Take the following situations:
More testing, less customer defects
Cost of testing: 10K (several issuses found)
Rework cost of internally found defects: 5K
Rework cost of externally found defects 3K
Less testing, more customer defects
Cost of testing: 2K (not many issues found)
Rework of issues (few ours + all from customer): 12K. (more twice as much as in the previous situation)
What I saved on testing I spent on fixing and even saved a bit.