Year	Number
1981	28.81719
1986	26.10579
1989	24.62188
1990	23.6807
1991	23.16132
1992	22.87984
1993	23.13682
1994	23.23862
1995	24.21648
1996	22.52481
1997	22.75129
1999	21.19649
2000	20.8769
2001	20.21471
2002	19.26334
2003	19.17128
2004	18.95654
2005	17.95097
2006	17.04615
2007	16.25435
2008	15.86201
2009	15.74791
2010	15.41452
2011	15.3188
2012	15.7877
2013	16.32995
2014	16.80874
2015	17.45665
2016	17.42007
2017	17.68211
2018	17.92314