Year	Number
1979	18.75201
1980	19.51886
1981	19.57202
1982	20.09736
1983	20.27909
1984	21.47174
1985	21.76367
1986	21.33819
1987	21.00523
1988	20.59424
1989	20.76528
1990	20.54936
1991	20.41168
1992	20.58884
1993	21.36472
1994	21.64064
1995	21.7949
1996	21.27554
1997	21.01355
1998	20.66331
1999	20.08408
2000	18.2523
2001	17.88083
2002	19.45608
2003	16.93675
2004	17.36133
2005	17.32658
2006	17.44748
2007	17.47822
2008	18.17297
2009	22.04076
2010	18.63684
2012	18.14681