Year	Number of
1990	1.42
1991	1.51
1992	1.61
1993	1.72
1994	1.84
1995	1.96
1996	2.06
1997	2.17
1998	2.28
1999	2.4
2000	2.53
2001	2.67
2002	2.82
2003	2.98
2004	3.15
2005	3.33
2006	3.52
2007	3.72
2008	3.94
2009	4.16
2010	4.41
2011	4.6
2012	4.81
2013	5.02
2014	5.24
2015	5.48
2016	5.72