Sentinel value
An ordinary-looking value used as a code for something else, most often 999, -1, or 1900-01-01 standing in for unknown.
Sentinels are dangerous because software treats them as real. They are included in every sum and average until someone remembers what they mean.
Where it comes up
- Disguised missing valuesA cell is not empty, but what it holds means there is no value: N/A, a dash, 999, or the word none.
See also
- Missing valueA cell where a value should exist and does not. The reason matters: not collected, not applicable, and lost are different situations.
- NullThe explicit marker a database or file uses for no value. It is different from zero, from an empty string, and from the text NULL typed into a cell.