Year	Number
1970	15.62344
1971	16.01635
1972	16.33902
1973	16.34304
1974	16.25088
1975	16.50779
1976	16.97802
1977	17.41123
1978	17.69278
1979	17.47271
1980	17.29536
1981	17.16362
1982	17.01328
1983	17.06694
1984	17.20238
1985	17.51538
1986	17.42906
1987	17.35877
1988	17.21338
1989	17.01695
1990	16.86998
1991	17.00194
1992	17.29041
1993	17.56601
1994	17.7796
1995	18.11501
1996	18.31698
1997	18.59721
1998	18.77217
1999	18.78857
2000	19.15325
2001	19.75936
2002	19.76352
2003	19.62639
2004	19.69354
2005	19.35036
2006	19.06011
2007	18.65244
2008	18.25977
2009	18.02258
2010	18.02609
2011	18.13249
2012	18.24865
2013	18.93456
2014	18.82211
2015	18.5601
2016	18.16581
2017	17.97486
2018	18.02335