| Value | Category |
|---|---|
| -9 | Don't know (spontaneous) |
| -8 | Refusal (spontaneous) |
| -7 | Does not apply |
| 1948 | |
| 1954 | |
| 1962 | |
| 1965 | |
| 1967 | |
| 1968 | |
| 1969 | |
| 1970 | |
| 1971 | |
| 1972 | |
| 1975 | |
| 1976 | |
| 1977 | |
| 1978 | |
| 1982 | |
| 1983 | |
| 1984 | |
| 1985 | |
| 1987 | |
| 1988 | |
| 1990 | |
| 1992 | |
| 1993 | |
| 1994 | |
| 1995 | |
| 1997 | |
| 1998 | |
| 1999 | |
| 2000 | |
| 2001 | |
| 2002 | |
| 2003 | |
| 2004 | |
| 2005 | |
| 2006 | |
| 2007 | |
| 2012 | |
| 2013 |