Year	Number of
1990	2.28
1991	2.37
1992	2.47
1993	2.58
1994	2.68
1995	2.78
1996	2.82
1997	2.83
1998	2.85
1999	2.9
2000	2.92
2001	2.98
2002	3.04
2003	3.09
2004	3.14
2005	3.17
2006	3.19
2007	3.22
2008	3.24
2009	3.25
2010	3.26
2011	3.27
2012	3.29
2013	3.32
2014	3.38
2015	3.48
2016	3.56