Year	Number of
1990	19.18
1991	18.82
1992	18.29
1993	17.64
1994	17.05
1995	16.5
1996	16.07
1997	16.28
1998	16.27
1999	15.82
2000	15.94
2001	15.85
2002	15.83
2003	16.06
2004	16.49
2005	17.15
2006	17.43
2007	17.82
2008	18.26
2009	18.79
2010	19.52
2011	20.18
2012	20.94
2013	21.78
2014	22.58
2015	23.37
2016	24.01