| Year | Value | | 2022 | 40.291829974807 | | 2021 | 30.518557632977 | | 2020 | 24.584440684476 | | 2019 | 32.854529067618 | | 2018 | 34.624505656025 | | 2017 | 30.259679298847 | | 2016 | 27.317567691058 | | 2015 | 25.750746981386 | | 2014 | 39.055791313495 | | 2013 | 44.855484297752 | | 2012 | 49.256219491329 | | 2011 | 50.972545871355 | | 2010 | 43.563651405824 | | 2009 | 36.598621488757 | | 2008 | 52.752269343227 | | 2007 | 48.92772775397 | | 2006 | 48.505527859401 | | 2005 | 48.653107962522 | | 2004 | 42.13973557448 | | 2003 | 34.816382256345 | | 2002 | 29.270317119371 | | 2001 | 28.027597577405 | | 2000 | 30.258205393145 | | 1999 | 25.621286827082 | | 1998 | 16.768094165837 | | 1997 | 22.762746792386 | | 1996 | 22.154593923915 | | 1995 | 19.802536381769 | | 1994 | 15.396782433546 | | 1993 | 15.508172768238 | | 1992 | 16.719143485154 | | 1991 | 8.5851149037527 | | 1990 | 15.192293445301 | | 1989 | 13.736455931333 | | 1988 | 16.64880073623 | | 1987 | 13.357770521168 | | 1986 | 13.62286797213 | | 1985 | 17.013199766182 | | 1984 | 19.671264105136 | | 1983 | 23.871153324904 | | 1982 | 32.906955309741 | | 1981 | 45.87318810185 | | 1980 | 50.413017458776 | | 1979 | 37.846631368557 | | 1978 | 34.275916301568 | | 1977 | 48.277971320015 | | 1976 | 60.332359369838 | | 1975 | 56.777301017784 | | 1974 | 67.27726625585 | | 1973 | 34.904997207248 | | 1972 | 36.498300172529 | | 1971 | 34.683825295268 |
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