Year	Number of
1990	3.69
1991	3.84
1992	3.99
1993	4.16
1994	4.36
1995	4.52
1996	4.75
1997	5.08
1998	5.35
1999	5.7
2000	5.85
2001	6.13
2002	6.4
2003	6.67
2004	6.94
2005	7.24
2006	7.6
2007	8.01
2008	8.37
2009	8.69
2010	8.93
2011	9.17
2012	9.39
2013	9.63
2014	9.89
2015	10.21
2016	10.5