Year	Number of
1990	5.08
1991	5.24
1992	5.42
1993	5.62
1994	5.78
1995	5.97
1996	6.04
1997	6.14
1998	6.16
1999	6.25
2000	6.31
2001	6.31
2002	6.37
2003	6.4
2004	6.49
2005	6.61
2006	6.69
2007	6.84
2008	6.97
2009	7.07
2010	7.26
2011	7.28
2012	7.25
2013	7.29
2014	7.4
2015	7.69
2016	7.81