Year	Number of
1990	3.4
1991	3.51
1992	3.61
1993	3.71
1994	3.82
1995	3.93
1996	3.99
1997	4.04
1998	4.12
1999	4.19
2000	4.27
2001	4.28
2002	4.3
2003	4.33
2004	4.36
2005	4.38
2006	4.38
2007	4.37
2008	4.36
2009	4.34
2010	4.33
2011	4.35
2012	4.38
2013	4.4
2014	4.44
2015	4.5
2016	4.55