Many people are approaching testing the year 2000 date change from a structural perspective, often at the unit test level. While this will validate the operation of a particular unit in isolation, the greater challenge is validating the combined effect of the changes. The ultimate test of the year 2000 changes is from the business or operational perspective. 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.