Year	Number of
1990	7.2
1991	6.89
1992	6.61
1993	6.34
1994	6.05
1995	5.77
1996	5.75
1997	5.77
1998	5.84
1999	5.92
2000	6.01
2001	6.08
2002	6.23
2003	6.41
2004	6.5
2005	6.62
2006	6.54
2007	6.62
2008	6.51
2009	6.66
2010	6.83
2011	7.08
2012	7.27
2013	7.38
2014	7.55
2015	7.6
2016	7.64