Year	Number of
1990	4.67
1991	4.73
1992	4.81
1993	4.78
1994	4.87
1995	4.91
1996	4.89
1997	5.21
1998	5.69
1999	5.91
2000	6.06
2001	6.02
2002	5.91
2003	5.76
2004	5.66
2005	5.49
2006	5.49
2007	5.36
2008	5.58
2009	5.67
2010	5.71
2011	5.81
2012	6.02
2013	6.15
2014	6.37
2015	6.53
2016	6.71