Year	Number of
1990	2.82
1991	2.88
1992	2.94
1993	3.01
1994	3.09
1995	3.16
1996	3.24
1997	3.32
1998	3.41
1999	3.5
2000	3.59
2001	3.67
2002	3.75
2003	3.84
2004	3.93
2005	4.04
2006	4.09
2007	4.13
2008	4.18
2009	4.24
2010	4.32
2011	4.36
2012	4.43
2013	4.5
2014	4.56
2015	4.63
2016	4.72