Year	Number
1970	19.16222
1971	19.27818
1972	19.13439
1973	19.00706
1974	18.8785
1975	18.79652
1976	18.82748
1977	18.70394
1978	18.62507
1979	18.55957
1980	18.32229
1981	18.24786
1982	18.10632
1983	17.96182
1984	17.77269
1985	17.60717
1986	17.43496
1987	17.4495
1988	17.34625
1989	17.24065
1990	17.13304
1991	16.97603
1992	16.88207
1993	16.76453
1994	16.46275
1995	16.52013
1996	16.47201
1997	16.37079
1998	16.45335
1999	16.26213
2000	15.92328
2001	15.88898
2002	15.56193
2003	15.48895
2004	15.15468
2005	14.92276
2006	14.72095
2007	14.52445
2008	14.35105
2009	14.29411
2010	14.02249
2011	14.13012
2012	14.1681
2013	14.13521
2014	14.11254
2015	14.02746
2016	13.90883
2017	13.94748
2018	14.03652