There are two problems with sentinel values. First, there may be no unused values. Using a negative value as a sentinel for age seems reasonable, but it doesn't work for a bank account, where negative values indicate debt. The second, and more insidious, problem with sentinel values is that they tend, over time, to become legitimate values as interpretations change and become more liberal, or as growth requires adding a few more digits to a product code.
This page last modified on 24 January 2006.