Year	Number of
1990	2.44
1991	2.49
1992	2.54
1993	2.6
1994	2.64
1995	2.7
1996	2.76
1997	2.83
1998	2.89
1999	2.99
2000	3.1
2001	3.21
2002	3.31
2003	3.42
2004	3.54
2005	3.62
2006	3.74
2007	3.84
2008	3.95
2009	4.03
2010	4.14
2011	4.26
2012	4.38
2013	4.53
2014	4.68
2015	4.82
2016	4.99