Year	Number
1973	13.30036
1974	13.24186
1975	14.34166
1976	15.04376
1977	14.89511
1978	15.10271
1979	15.58191
1980	14.76027
1981	14.38787
1982	14.44842
1983	14.57822
1984	14.48336
1985	13.85942
1986	14.45351
1987	14.83944
1988	14.90368
1989	14.98924
1990	15.04864
1991	14.91576
1992	14.7481
1993	14.80328
1994	14.80501
1995	14.79794
1996	14.87552
1997	15.16173
1998	15.50061
1999	15.72794
2000	15.72283
2001	15.96918
2002	16.0677
2003	16.25071
2004	16.36972
2005	16.376
2006	16.29897
2007	16.24158
2008	16.17324
2009	15.76697
2010	23.68782
2011	24.38425
2012	22.75671
2013	22.38606
2014	22.48774
2015	19.65367
2016	20.14287
2017	20.63001
2018	19.1202