Year	Number of
1990	2.6
1991	2.59
1992	2.57
1993	2.52
1994	2.49
1995	2.47
1996	2.45
1997	2.41
1998	2.4
1999	2.35
2000	2.32
2001	2.29
2002	2.24
2003	2.16
2004	2.08
2005	2.04
2006	2.01
2007	1.93
2008	1.9
2009	1.93
2010	1.99
2011	1.99
2012	2
2013	2.07
2014	2.09
2015	2.12
2016	2.18