Year	Number of
1990	6.38
1991	6.24
1992	6.16
1993	6.06
1994	5.9
1995	5.89
1996	5.92
1997	5.78
1998	5.82
1999	5.52
2000	5.56
2001	5.3
2002	5.24
2003	5.23
2004	5.16
2005	5.11
2006	5.18
2007	5.07
2008	5.2
2009	5.24
2010	5.43
2011	5.61
2012	5.75
2013	5.86
2014	6.34
2015	6.56
2016	6.88