Year	Number of
1990	4.29
1991	4.35
1992	4.4
1993	4.47
1994	4.47
1995	4.51
1996	4.53
1997	4.55
1998	4.59
1999	4.64
2000	4.69
2001	4.8
2002	4.98
2003	5.16
2004	5.37
2005	5.52
2006	5.77
2007	5.99
2008	6.19
2009	6.35
2010	6.56
2011	6.77
2012	6.99
2013	7.23
2014	7.48
2015	7.71
2016	7.97