Year	Number of
1990	2.35
1991	2.37
1992	2.4
1993	2.43
1994	2.48
1995	2.53
1996	2.46
1997	2.37
1998	2.31
1999	2.27
2000	2.24
2001	2.2
2002	2.15
2003	2.11
2004	2.07
2005	2.03
2006	2.04
2007	2.05
2008	2.07
2009	2.09
2010	2.11
2011	2.16
2012	2.23
2013	2.29
2014	2.36
2015	2.43
2016	2.62