Year	Number of
1990	4.99
1991	5.02
1992	4.99
1993	4.88
1994	4.83
1995	4.87
1996	4.93
1997	4.95
1998	5.12
1999	5.41
2000	5.67
2001	5.83
2002	5.91
2003	6.15
2004	6.39
2005	6.43
2006	6.37
2007	6.36
2008	6.42
2009	6.62
2010	6.86
2011	6.72
2012	6.74
2013	7.01
2014	7.38
2015	7.69
2016	7.99