Year	Number of
1990	2.22
1991	2.29
1992	2.4
1993	2.48
1994	2.57
1995	2.66
1996	2.74
1997	2.81
1998	2.89
1999	2.94
2000	3.01
2001	3.08
2002	3.18
2003	3.29
2004	3.42
2005	3.57
2006	3.7
2007	3.82
2008	3.91
2009	4.03
2010	4.17
2011	4.37
2012	4.54
2013	4.81
2014	5.07
2015	5.33
2016	5.37