Year	Number of
1990	3.21
1991	3.28
1992	3.35
1993	3.44
1994	3.53
1995	3.62
1996	3.71
1997	3.8
1998	3.89
1999	3.99
2000	4.09
2001	4.14
2002	4.2
2003	4.26
2004	4.32
2005	4.38
2006	4.43
2007	4.47
2008	4.5
2009	4.53
2010	4.55
2011	4.56
2012	4.57
2013	4.56
2014	4.56
2015	4.57
2016	4.64