Year	Number of
1990	5.18
1991	5.43
1992	5.67
1993	5.91
1994	6.19
1995	6.48
1996	6.76
1997	7.04
1998	7.31
1999	7.4
2000	7.45
2001	7.48
2002	7.55
2003	7.7
2004	7.87
2005	8.02
2006	8.15
2007	8.38
2008	8.6
2009	8.75
2010	8.87
2011	8.98
2012	9.12
2013	9.28
2014	9.42
2015	9.58
2016	9.72