Year	Number of
1990	3.32
1991	3.39
1992	3.46
1993	3.56
1994	3.62
1995	3.72
1996	3.84
1997	3.92
1998	4.04
1999	4.14
2000	4.23
2001	4.29
2002	4.35
2003	4.43
2004	4.49
2005	4.56
2006	4.62
2007	4.64
2008	4.71
2009	4.77
2010	4.78
2011	4.81
2012	4.89
2013	4.85
2014	4.83
2015	4.8
2016	4.85