| Value | Category |
|---|---|
| 1 | month and year - information complete |
| 2 | month and age - year imputed |
| 3 | year and age - month imputed |
| 4 | year and age - year ignored |
| 5 | year - age/month imputed |
| 6 | age - year/month imputed |
| 7 | month - age/year imputed |
| 8 | none - all imputed |
| Sysmiss |