Year	Number of
1990	3.74
1991	3.94
1992	4.09
1993	3.97
1994	3.96
1995	4.09
1996	3.93
1997	4.04
1998	3.99
1999	3.95
2000	4.06
2001	4.11
2002	4.12
2003	4.21
2004	4.21
2005	4.25
2006	4.51
2007	4.66
2008	4.83
2009	5.01
2010	5.23
2011	5.4
2012	5.66
2013	5.89
2014	6.09
2015	6.4
2016	6.7