Year	Number of
1990	4.83
1991	4.87
1992	4.91
1993	4.94
1994	4.99
1995	5.03
1996	5.01
1997	4.99
1998	4.97
1999	4.97
2000	4.95
2001	5.02
2002	5.09
2003	5.17
2004	5.25
2005	5.33
2006	5.37
2007	5.47
2008	5.55
2009	5.67
2010	5.8
2011	5.94
2012	6.01
2013	6.07
2014	6.13
2015	6.17
2016	6.13