Year	Number of
1990	3.05
1991	3.13
1992	3.21
1993	3.29
1994	3.37
1995	3.46
1996	3.53
1997	3.61
1998	3.69
1999	3.77
2000	3.84
2001	3.89
2002	3.94
2003	3.99
2004	4.05
2005	4.13
2006	4.19
2007	4.25
2008	4.31
2009	4.38
2010	4.46
2011	4.56
2012	4.67
2013	4.8
2014	4.92
2015	5.05
2016	5.21