Year	Number of
1990	3.42
1991	3.48
1992	3.55
1993	3.62
1994	3.71
1995	3.8
1996	3.87
1997	3.95
1998	4.05
1999	4.16
2000	4.28
2001	4.32
2002	4.37
2003	4.44
2004	4.51
2005	4.6
2006	4.75
2007	4.9
2008	5.08
2009	5.25
2010	5.44
2011	5.73
2012	6.05
2013	6.39
2014	6.76
2015	7.14
2016	7.55