Year	Number of
1990	3.13
1991	3.15
1992	3.18
1993	3.21
1994	3.25
1995	3.33
1996	3.34
1997	3.28
1998	3.32
1999	3.22
2000	3.28
2001	3.07
2002	3.05
2003	3.04
2004	3.02
2005	3.04
2006	3.02
2007	2.96
2008	2.95
2009	2.99
2010	3.13
2011	3.19
2012	3.42
2013	3.64
2014	3.92
2015	4.41
2016	4.34