Year	Number
1970	31.49753
1971	31.10691
1972	31.13544
1973	31.6316
1974	31.86033
1975	31.88716
1976	31.69432
1977	31.09203
1978	30.41595
1979	29.92445
1980	29.70457
1981	29.63527
1982	29.79913
1983	29.46045
1984	29.73759
1985	30.27653
1986	30.38968
1987	30.44868
1988	30.12058
1989	30.17571
1990	30.3123
1991	30.15139
1992	29.79699
1993	29.35239
1994	29.47662
1995	29.49281
1996	29.35192
1997	29.37182
1998	29.47893
1999	29.99965
2000	29.7809
2001	29.83365
2002	29.11413
2003	28.38542
2004	28.27827
2005	27.47959
2006	27.20267
2007	26.57839
2008	25.96882
2009	25.20532
2010	24.98201
2011	24.53034
2012	23.76045
2013	23.67238
2014	23.97824
2015	23.55641
2016	23.2841
2017	23.05043
2018	22.81707