How are missing values, “blank but applicable”, “don know” and other values coded?
There are codes for refused (7-fill: that is 7, or 77, or 777…, depending on the number of digits required for a particular data value), don’t know (9-fill), and missing values (a blank field) which means the person was not asked the question or given the test. There is no longer a specific code for those cases where the variable response is ” blank but applicable”; for such cases the values are designated as missing values. For laboratory data there are special considerations. When a laboratory value was less than the lower limit of detection (LOD), a ” fill” value based on the LOD was used instead of the sample value as the sample value was deemed ” not detectable.” An indicator variable taking value (0 or 1) is used to identify which values are real and which values are fill values.
