Year	Number
1971	14.47284
1972	14.6806
1975	18.37426
1976	17.31037
1977	16.27757
1978	16.84725
1981	17.68173
1982	17.65963
1983	18.03304
1984	17.94618
1985	17.45694
1986	17.19945
1987	17.23617
1988	17.53487
1989	17.28767
1990	16.93628
1991	16.65771
1992	16.47229
1993	16.43185
1994	16.3721
1995	16.20095
1996	16.22758
1997	16.31588
1998	16.92203
1999	16.80328
2000	16.92285
2001	16.8772
2002	16.96424
2003	17.14063
2004	17.94653
2005	17.80164
2006	17.83042
2007	17.91581
2008	18.00744
2009	17.63968
2010	17.91261
2011	17.67731
2012	17.74098
2013	17.95597
2014	16.12339
2015	16.08213
2016	16.30684
2017	16.85759