Year	Number of
1990	4.36
1991	4.46
1992	4.55
1993	4.65
1994	4.77
1995	4.91
1996	4.88
1997	4.86
1998	4.87
1999	4.88
2000	4.9
2001	5.01
2002	5.12
2003	5.25
2004	5.38
2005	5.52
2006	5.65
2007	5.78
2008	5.93
2009	6.1
2010	6.27
2011	6.57
2012	6.89
2013	7.23
2014	7.59
2015	7.95
2016	8.33