Year	Number of
1990	4.07
1991	4.19
1992	4.3
1993	4.39
1994	4.48
1995	4.56
1996	4.6
1997	4.64
1998	4.67
1999	4.67
2000	4.64
2001	4.68
2002	4.73
2003	4.8
2004	4.88
2005	4.97
2006	5.03
2007	5.13
2008	5.21
2009	5.34
2010	5.48
2011	5.65
2012	5.77
2013	5.88
2014	6
2015	6.12
2016	6.18