Year	Number of
1990	4.49
1991	4.54
1992	4.65
1993	4.77
1994	4.9
1995	5.04
1996	5.06
1997	5.13
1998	5.32
1999	5.63
2000	5.97
2001	5.87
2002	5.88
2003	5.95
2004	5.94
2005	6.02
2006	6.04
2007	5.97
2008	6.01
2009	6.16
2010	6.21
2011	6.28
2012	6.53
2013	6.88
2014	7.29
2015	7.63
2016	7.71