Year	Number of
1990	4.69
1991	4.86
1992	5.03
1993	5.19
1994	5.36
1995	5.54
1996	5.73
1997	5.93
1998	6.13
1999	6.31
2000	6.48
2001	6.63
2002	6.79
2003	6.95
2004	7.15
2005	7.36
2006	7.54
2007	7.72
2008	7.89
2009	8.08
2010	8.29
2011	8.51
2012	8.76
2013	9.01
2014	9.26
2015	9.51
2016	9.91