Year	Number of
1990	4.31
1991	4.36
1992	4.42
1993	4.47
1994	4.48
1995	4.51
1996	4.6
1997	4.74
1998	4.94
1999	5.25
2000	5.48
2001	5.7
2002	5.86
2003	6.13
2004	6.37
2005	6.55
2006	6.55
2007	6.57
2008	6.58
2009	6.61
2010	6.67
2011	6.94
2012	7.14
2013	7.24
2014	7.29
2015	7.28
2016	7.39