Year	Number of
1990	4.34
1991	4.7
1992	5.01
1993	5.35
1994	5.59
1995	5.53
1996	5.53
1997	5.59
1998	5.67
1999	5.72
2000	5.75
2001	5.78
2002	5.85
2003	6.08
2004	6.59
2005	6.86
2006	7.17
2007	7.51
2008	7.84
2009	8.19
2010	8.41
2011	8.58
2012	8.74
2013	8.88
2014	9.02
2015	9.16
2016	9.2