| Value | Category | 
|---|---|
| -9 | Don't know (spontaneous) | 
| -8 | Refusal (spontaneous) | 
| -7 | Does not apply | 
| 1897 | |
| 1940 | |
| 1946 | |
| 1960 | |
| 1967 | |
| 1968 | |
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| 1972 | |
| 1975 | |
| 1978 | |
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| 1982 | |
| 1983 | |
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| 1986 | |
| 1987 | |
| 1988 | |
| 1989 | |
| 1990 | |
| 1991 | |
| 1992 | |
| 1993 | |
| 1994 | |
| 1995 | |
| 1996 | |
| 1997 | |
| 1998 | |
| 1999 | |
| 2000 | |
| 2001 | |
| 2002 | |
| 2003 | |
| 2004 | |
| 2005 | |
| 2006 | |
| 2007 | |
| 2008 | |
| 2009 | |
| 2010 | |
| 2011 | |
| 2012 | |
| 2013 | |
| 2014 | |
| Sysmiss |