| Value | Category | Cases | |
|---|---|---|---|
| 1 | Before 1970 | 5374 |
17.8%
|
| 2 | 1970s | 3499 |
11.6%
|
| 3 | 1980s | 5563 |
18.4%
|
| 4 | 1990s | 8436 |
27.9%
|
| 5 | 2000s | 1425 |
4.7%
|
| 11 | Don't Know | 5965 |
19.7%
|