| Value | Category | 
|---|---|
| 1 | less than one week | 
| 2 | 1 week or more but less than a month | 
| 3 | 1 month or more but less than 3 months | 
| 4 | 3 months or more but less than 6 months | 
| 5 | 6 months or more but less than 1 year | 
| 6 | 1 year or more but less than 2 years | 
| 7 | 2 years or more | 
| 8 | duration is unspecified | 
| Sysmiss |