Year	Number of
1990	3.76
1991	3.74
1992	3.78
1993	3.81
1994	3.99
1995	4.32
1996	4.5
1997	4.56
1998	4.66
1999	4.78
2000	4.92
2001	5.02
2002	5.22
2003	5.47
2004	5.66
2005	5.78
2006	5.9
2007	6.02
2008	6.19
2009	6.3
2010	6.47
2011	6.68
2012	6.95
2013	7.2
2014	7.33
2015	7.31
2016	7.5