Year	Number of
1990	3.05
1991	3.13
1992	3.23
1993	3.33
1994	3.43
1995	3.56
1996	3.65
1997	3.75
1998	3.82
1999	3.87
2000	3.93
2001	3.96
2002	4
2003	4.06
2004	4.09
2005	4.17
2006	4.27
2007	4.35
2008	4.48
2009	4.57
2010	4.71
2011	4.88
2012	5.06
2013	5.22
2014	5.4
2015	5.59
2016	5.81