Year	Number
1970	19.74295
1971	19.82311
1972	19.65779
1973	19.49736
1974	19.32231
1975	19.20083
1976	19.22192
1977	19.06723
1978	18.95499
1979	18.88196
1980	18.62054
1981	18.57203
1982	18.43409
1983	18.30181
1984	18.10332
1985	17.93051
1986	17.76823
1987	17.79318
1988	17.67651
1989	17.57816
1990	17.47583
1991	17.32986
1992	17.22885
1993	17.11483
1994	16.76915
1995	16.86978
1996	16.77798
1997	16.6662
1998	16.7547
1999	16.55088
2000	16.14926
2001	16.10634
2002	15.69605
2003	15.59026
2004	15.29514
2005	15.12869
2006	14.8939
2007	14.7229
2008	14.60831
2009	14.54785
2010	14.27188
2011	14.38285
2012	14.45945
2013	14.52542
2014	14.45057
2015	14.30129
2016	14.13223
2017	14.10818
2018	14.12635