Year	Number of
1990	3.61
1991	3.67
1992	3.75
1993	3.84
1994	3.86
1995	4.01
1996	4.2
1997	4.37
1998	4.61
1999	4.71
2000	4.8
2001	4.9
2002	5
2003	5.09
2004	5.17
2005	5.25
2006	5.34
2007	5.3
2008	5.37
2009	5.47
2010	5.44
2011	5.53
2012	5.85
2013	5.85
2014	5.92
2015	6.01
2016	6.13