Year	Number of
1990	4.36
1991	4.38
1992	4.39
1993	4.42
1994	4.45
1995	4.46
1996	4.46
1997	4.44
1998	4.43
1999	4.43
2000	4.43
2001	4.44
2002	4.47
2003	4.5
2004	4.54
2005	4.59
2006	4.58
2007	4.58
2008	4.59
2009	4.62
2010	4.67
2011	4.71
2012	4.76
2013	4.82
2014	4.89
2015	4.96
2016	5.04