Year	Number of
1990	4.17
1991	4.47
1992	4.63
1993	4.76
1994	4.86
1995	5.03
1996	5.06
1997	5.12
1998	5.19
1999	5.51
2000	5.62
2001	5.57
2002	5.34
2003	5.33
2004	5.47
2005	5.53
2006	5.57
2007	5.68
2008	6.01
2009	6.33
2010	6.66
2011	6.83
2012	6.85
2013	6.85
2014	7.18
2015	7.31
2016	7.64