Year	Number of
1990	19.05
1991	19.3
1992	19.56
1993	18.79
1994	18.03
1995	17.46
1996	17.44
1997	17.36
1998	17.56
1999	17.82
2000	17.84
2001	17.94
2002	17.86
2003	18.01
2004	18.09
2005	18.01
2006	17.5
2007	17.23
2008	16.79
2009	16.33
2010	16.08
2011	16.46
2012	16.64
2013	16.03
2014	15.62
2015	15.39
2016	15.39