Year	Number of
1990	2.89
1991	2.96
1992	3.02
1993	3.06
1994	3.13
1995	3.21
1996	3.31
1997	3.37
1998	3.47
1999	3.53
2000	3.6
2001	3.65
2002	3.68
2003	3.67
2004	3.65
2005	3.67
2006	3.69
2007	3.66
2008	3.69
2009	3.83
2010	4.01
2011	4.08
2012	4.18
2013	4.39
2014	4.52
2015	4.66
2016	4.86