Year	Number of
1990	3.14
1991	3.27
1992	3.4
1993	3.53
1994	3.64
1995	3.75
1996	3.84
1997	3.94
1998	4.03
1999	4.14
2000	4.23
2001	4.36
2002	4.52
2003	4.69
2004	4.87
2005	5.05
2006	5.25
2007	5.46
2008	5.66
2009	5.89
2010	6.13
2011	6.34
2012	6.57
2013	6.84
2014	7.11
2015	7.37
2016	7.57