Year	Number of
1990	17.82
1991	18.4
1992	18.91
1993	18.78
1994	19.6
1995	20.04
1996	18.83
1997	17.89
1998	17.33
1999	16.38
2000	16.9
2001	16.19
2002	16.26
2003	16.47
2004	16.52
2005	15.99
2006	15.91
2007	15.42
2008	15.54
2009	15.65
2010	15.64
2011	15.58
2012	15.64
2013	15.84
2014	15.87
2015	15.91
2016	15.07