Year	Number of
1990	2.41
1991	2.51
1992	2.62
1993	2.73
1994	2.84
1995	2.96
1996	2.84
1997	2.92
1998	2.85
1999	2.81
2000	2.85
2001	2.83
2002	2.82
2003	2.85
2004	2.84
2005	2.91
2006	3.01
2007	3.07
2008	3.13
2009	3.2
2010	3.31
2011	3.45
2012	3.59
2013	3.75
2014	3.81
2015	3.91
2016	4.04