Year	Number
1971	21.53908
1972	21.67261
1973	21.74492
1974	22.11884
1975	22.44474
1976	22.71847
1977	22.93493
1978	23.12388
1979	23.16096
1980	23.48639
1981	23.44852
1982	23.36964
1983	23.28801
1984	23.18164
1985	23.24084
1986	23.39976
1987	22.73828
1988	22.36807
1989	22.24505
1990	22.42464
1991	22.31839
1992	22.22583
1993	22.1962
1994	22.58269
1995	22.90159
1996	22.71646
1997	22.65817
1998	22.86113
1999	22.94543
2000	23.44485
2001	23.31164
2002	23.6356
2003	23.55155
2004	24.15766
2005	23.88063
2006	23.92678
2007	23.63487
2008	23.69813
2009	23.30246
2010	23.24454
2011	23.31242
2012	23.04886
2013	22.07264
2014	21.88557
2015	21.44457
2016	21.1948
2017	20.97459
2018	21.37058