With so many systems poorly documented, validating that the system will meet business needs will require both a rigorous test approach and the ability to reverse engineer tests. Past estimates for changing existing systems to handle the year 2000 have been doubled to account for the costs of testing. Testing is inherently costly. My research shows that organizations spend anywhere from 30 to 50 percent of the development effort on test-related activities. One factor that contributes to the high cost of testing is the time involved to plan and perform an adequate test.
Some organizations skip the planning part and think that random testing takes less time.
In reality random testing is often redundant, since you never know for sure what has already been tested. In addition, random testing misses some of the more elusive defects in a system. Test execution is costly when all of the tests are done manually. Not only is manual testing imprecise, it is slow. It takes time to manually perform the tests, examine the results and then report the defects.