Year	Number of
1990	9.87
1991	9.85
1992	9.74
1993	9.55
1994	9.37
1995	9.21
1996	8.98
1997	9.08
1998	9.24
1999	9.15
2000	9.48
2001	9.78
2002	10.16
2003	10.75
2004	11.25
2005	11.87
2006	12.34
2007	12.85
2008	13.35
2009	14.01
2010	14.63
2011	15.31
2012	16.11
2013	16.94
2014	17.81
2015	18.67
2016	19.58