Year	Number of
1990	3.36
1991	3.33
1992	3.39
1993	3.4
1994	3.4
1995	3.49
1996	3.5
1997	3.44
1998	3.43
1999	3.52
2000	3.68
2001	3.66
2002	3.61
2003	3.69
2004	3.7
2005	3.73
2006	3.82
2007	3.77
2008	3.85
2009	3.94
2010	4.01
2011	4.07
2012	4.11
2013	4.12
2014	4.35
2015	4.4
2016	4.51