Year	Number of
1990	1.22
1991	1.28
1992	1.35
1993	1.42
1994	1.49
1995	1.57
1996	1.65
1997	1.74
1998	1.83
1999	1.92
2000	2.02
2001	2.09
2002	2.15
2003	2.22
2004	2.29
2005	2.37
2006	2.47
2007	2.57
2008	2.68
2009	2.8
2010	2.92
2011	3.09
2012	3.27
2013	3.47
2014	3.67
2015	3.9
2016	4.13