| Value | Category |
|---|---|
| 1 | Skill level 1 (low) |
| 2 | Skill level 2 (medium) |
| 3 | Skill levels 3 and 4 (high) |
| 4 | Not elsewhere classified |
| a | Don't know |
| b | Refused to answer |
| c | Outlier set to missing |
| d | Dropped during anonymization (available on request) |
| e | Skipped in error |
| z | Not applicable |