Year	Number of
1990	1.57
1991	1.65
1992	1.73
1993	1.81
1994	1.88
1995	1.95
1996	2.01
1997	2.07
1998	2.13
1999	2.19
2000	2.25
2001	2.33
2002	2.42
2003	2.51
2004	2.62
2005	2.73
2006	2.85
2007	2.98
2008	3.12
2009	3.26
2010	3.42
2011	3.57
2012	3.74
2013	3.94
2014	4.13
2015	4.32
2016	4.5