Year	Number of
1990	5.46
1991	5.48
1992	5.49
1993	5.52
1994	5.53
1995	5.53
1996	5.51
1997	5.46
1998	5.44
1999	5.41
2000	5.37
2001	5.37
2002	5.38
2003	5.41
2004	5.45
2005	5.51
2006	5.5
2007	5.52
2008	5.53
2009	5.59
2010	5.67
2011	5.73
2012	5.8
2013	5.89
2014	5.99
2015	6.09
2016	6.18