Year	Number
1977	21.30458
1978	20.88014
1979	20.95965
1980	21.21861
1981	22.37284
1982	22.59032
1983	22.24806
1984	22.43749
1985	21.93786
1986	21.6638
1987	21.30776
1988	21.28021
1989	21.94747
1990	21.42634
1991	21.24569
1992	20.77807
1993	20.33242
1994	20.34836
1995	20.28509
1996	19.88794
1997	19.39827
1998	19.20179
1999	17.0156
2000	14.49546
2001	15.79078
2002	15.51835
2003	15.60349
2004	15.32752
2005	15.08635
2006	15.65369
2007	15.79384
2008	15.89272
2009	16.91442
2010	17.26698
2011	18.19664
2012	19.23137
2013	19.63626
2014	18.60531
2015	19.25838
2016	19.86063
2017	19.64509
2018	19.58741