Year	Number of
1990	3.3
1991	3.36
1992	3.4
1993	3.45
1994	3.54
1995	3.66
1996	3.8
1997	3.83
1998	3.77
1999	3.97
2000	3.95
2001	4.02
2002	4.17
2003	4.23
2004	4.23
2005	4.22
2006	4.32
2007	4.42
2008	4.51
2009	4.57
2010	4.72
2011	4.82
2012	4.92
2013	5.24
2014	5.56
2015	5.58
2016	5.71