Year	Number of
1990	3.51
1991	3.47
1992	3.53
1993	3.53
1994	3.54
1995	3.6
1996	3.62
1997	3.55
1998	3.55
1999	3.64
2000	3.85
2001	3.8
2002	3.75
2003	3.81
2004	3.84
2005	3.88
2006	3.96
2007	3.9
2008	3.97
2009	4.04
2010	4.1
2011	4.15
2012	4.18
2013	4.16
2014	4.38
2015	4.41
2016	4.5