| Value | Category | 
|---|---|
| 1 | Less than an month | 
| 2 | 1 month to less than 2 months | 
| 3 | 2 months to less than 3 months | 
| 4 | 3 months to less than 4 months | 
| 5 | 4 months to less than 6 months | 
| 6 | 6 months to less than 1 year | 
| 7 | 1 year to less than 3 years | 
| 8 | 3 years or more | 
| 9 | Don't known | 
| 88 | Not applicable | 
| 99 | Unspecified |