It might take 30 minutes to test a software component by hand with a few simple test cases. Do it! I am suggesting this in addition to, not in lieu of, the more thorough computer-based unit testing that is already being performed. What is the cost? Just 30 minutes. What is the alternative? Save 30 minutes now, proceed with unit, integration, and system testing. The system fails. Three to four person days are spent trying to isolate the cause of the failure. A half-dozen components are isolated as possible candidates. Each is given to its developers for further examination. Each candidate spends 30 minutes testing the component by hand with a few simple test cases. In short, 30 minutes are less than three to four person days plus 6 x 30 minutes.
Another reason to perform routine manual testing is that it identifies components that need to be updated. All your builds and automated tests will pass and not complain with outdated tech. But when developers have to manually test a component with an outdated build or libraries, they will complain and get it updated. Which will keep the software relevant.
Reference:
Ledgard, H., Programming Proverbs, Rochelle Park, NJ: Hayden Book Company, 1975.
