Year	Number
1972	21.73703
1973	21.76131
1974	21.76084
1975	23.07027
1976	22.85298
1977	22.4018
1978	22.36968
1979	21.76458
1980	21.96468
1981	22.10791
1982	23.05265
1983	22.71121
1984	23.40725
1985	23.35344
1986	23.19512
1987	23.33911
1988	23.19729
1989	23.066
1990	23.081
1991	22.48302
1992	22.65729
1993	22.63316
1994	22.19683
1995	22.22028
1996	21.59139
1997	21.3258
1998	20.69674
1999	20.32186
2000	19.21318
2001	19.61739
2002	20.27564
2003	19.71392
2004	19.94354
2005	19.55811
2006	20.06499
2007	20.09221
2008	20.81789
2009	21.39703
2010	21.17729
2011	19.97494
2012	18.71089
2013	18.41827
2014	19.31791
2015	19.63288
2016	20.34415
2017	20.30969
2018	20.95591