Year	Number
1971	31.70258
1972	31.95864
1973	30.18927
1974	28.99599
1975	27.2405
1976	26.19217
1977	26.22558
1978	24.09281
1979	23.56056
1980	22.87474
1981	22.4016
1982	21.97739
1983	22.21713
1984	22.26809
1985	21.70169
1986	21.01602
1987	20.00605
1988	19.54747
1989	18.15843
1990	17.06464
1991	16.25429
1992	16.11723
1993	16.50458
1994	15.81167
1995	15.5598
1996	14.92441
1997	15.10365
1998	15.85298
1999	15.90173
2000	15.81784
2001	14.50301
2002	13.94541
2003	13.21774
2004	12.74668
2005	12.48933
2006	12.35586
2007	11.92062
2013	11.91166
2014	11.7786
2015	10.89572
2016	10.72371
2017	10.43688