Year	Number of
1990	25.79
1991	25.41
1992	25.04
1993	24.66
1994	24.33
1995	24.06
1996	22.72
1997	21.5
1998	20.36
1999	19.31
2000	18.33
2001	17.89
2002	17.48
2003	17.12
2004	16.77
2005	16.45
2006	16.89
2007	17.36
2008	17.85
2009	18.39
2010	18.97
2011	19.69
2012	20.46
2013	21.26
2014	22.09
2015	22.96
2016	23.92