Year	Number of
1990	3.46
1991	3.53
1992	3.61
1993	3.68
1994	3.76
1995	3.83
1996	3.9
1997	3.98
1998	4.05
1999	4.15
2000	4.24
2001	4.34
2002	4.43
2003	4.52
2004	4.62
2005	4.71
2006	4.78
2007	4.85
2008	4.91
2009	5
2010	5.1
2011	5.18
2012	5.26
2013	5.36
2014	5.45
2015	5.54
2016	5.66