Year	Number of
1990	1.64
1991	1.73
1992	1.79
1993	1.86
1994	1.92
1995	1.99
1996	2.05
1997	2.12
1998	2.2
1999	2.31
2000	2.42
2001	2.55
2002	2.7
2003	2.85
2004	3.01
2005	3.18
2006	3.34
2007	3.43
2008	3.53
2009	3.63
2010	3.73
2011	3.83
2012	3.93
2013	4.03
2014	4.13
2015	4.23
2016	4.38