Year	Number of
1990	2.4
1991	2.51
1992	2.61
1993	2.72
1994	2.83
1995	2.93
1996	2.97
1997	3
1998	3.02
1999	3.05
2000	3.06
2001	3.11
2002	3.17
2003	3.21
2004	3.26
2005	3.34
2006	3.37
2007	3.41
2008	3.43
2009	3.48
2010	3.51
2011	3.52
2012	3.53
2013	3.54
2014	3.59
2015	3.63
2016	3.66