Year	Number
1972	25.24272
1973	25.629
1974	26.27212
1975	27.28375
1976	27.53304
1977	27.97247
1978	27.95924
1979	27.72425
1980	27.8436
1981	27.85061
1982	28.84217
1983	28.18262
1984	27.95059
1985	26.98542
1986	26.53976
1987	25.90829
1988	25.61988
1989	25.18172
1990	24.33747
1991	23.7869
1992	23.20851
1993	22.40932
1994	21.65855
1995	21.33749
1996	20.73777
1997	20.04093
1998	20.28937
1999	18.58187
2000	15.73946
2001	17.04151
2002	16.36512
2003	16.2897
2004	16.12309
2005	15.95855
2006	16.43614
2007	17.16056
2008	17.34071
2009	18.33015
2010	18.63465
2011	19.22535
2012	19.99454
2013	19.90951
2014	19.63613
2015	20.02342
2016	20.04403
2017	20.25436
2018	19.77285