Year	Number of
1990	2.52
1991	2.54
1992	2.55
1993	2.54
1994	2.54
1995	2.55
1996	2.6
1997	2.73
1998	2.86
1999	3
2000	3.17
2001	3.29
2002	3.4
2003	3.53
2004	3.68
2005	3.86
2006	3.97
2007	4.1
2008	4.24
2009	4.38
2010	4.56
2011	4.7
2012	4.87
2013	5.05
2014	5.22
2015	5.4
2016	5.55