Year	Number of
1990	3.5
1991	3.59
1992	3.69
1993	3.78
1994	3.89
1995	3.99
1996	4.05
1997	4.11
1998	4.17
1999	4.22
2000	4.25
2001	4.32
2002	4.39
2003	4.48
2004	4.58
2005	4.67
2006	4.75
2007	4.83
2008	4.92
2009	5.01
2010	5.12
2011	5.23
2012	5.36
2013	5.48
2014	5.6
2015	5.73
2016	5.87