Year	Number of
1990	3.92
1991	3.95
1992	3.99
1993	4.05
1994	4.01
1995	4.03
1996	3.98
1997	3.86
1998	3.83
1999	3.67
2000	3.66
2001	3.47
2002	3.45
2003	3.42
2004	3.38
2005	3.37
2006	3.46
2007	3.5
2008	3.54
2009	3.58
2010	3.78
2011	3.77
2012	3.91
2013	4.02
2014	4.16
2015	4.19
2016	4.27