| Year | |
|---|---|
| 1978 | 8.5 |
| 1979 | 9.0 |
| 1980 | 9.3 |
| 1981 | 10.8 |
| 1982 | 12.4 |
| 1983 | 9.8 |
| 1984 | 5.2 |
| 1985 | 3.9 |
| 1986 | 4.1 |
| 1987 | 4.2 |
| 1988 | 4.4 |
| 1989 | 4.0 |
| 1990 | 5.3 |
| 1991 | 5.0 |
| 1992 | 4.4 |
| 1993 | 1.6 |
| 1994 | 1.5 |
| 1995 | 0.4 |
| 1996 | 2.2 |
| 1997 | 1.7 |
| 1998 | 1.3 |
| 1999 | 0.9 |
| 2000 | 2.3 |
| 2001 | 2.7 |
| 2002 | 2.3 |
| 2003 | 3.0 |
| 2004 | 1.9 |
| 2005 | 2.1 |
| 2006 | 2.3 |
| 2007 | 1.9 |
| 2008 | 2.1 |
| 2009 | 2.2 |
| 2010 | 0.4 |
| 2011 | 2.0 |
| 2012 | 2.9 |
| 2013 | 1.2 |
| 2014 | 1.0 |
| 2015 | 1.9 |
| 2016 | 1.2 |
| 2017 | 1.5 |
| 2018 | 1.6 |
| 2019 | 2.1 |