Year	Number of
1990	4.64
1991	4.82
1992	5
1993	5.17
1994	5.36
1995	5.56
1996	5.76
1997	5.96
1998	6.16
1999	6.25
2000	6.34
2001	6.39
2002	6.46
2003	6.57
2004	6.69
2005	6.79
2006	6.88
2007	7.02
2008	7.15
2009	7.25
2010	7.33
2011	7.4
2012	7.47
2013	7.58
2014	7.67
2015	7.77
2016	7.88