Year	Number of
1990	1.21
1991	1.28
1992	1.35
1993	1.43
1994	1.51
1995	1.6
1996	1.65
1997	1.71
1998	1.77
1999	1.79
2000	1.85
2001	1.87
2002	1.92
2003	2.01
2004	2.13
2005	2.25
2006	2.38
2007	2.47
2008	2.55
2009	2.67
2010	2.79
2011	2.95
2012	3.1
2013	3.28
2014	3.49
2015	3.71
2016	3.94