Year	Number of
1990	18.84
1991	18.86
1992	18.76
1993	17.94
1994	17.98
1995	16.9
1996	16.95
1997	16.65
1998	16.3
1999	15.94
2000	15.87
2001	15.39
2002	15.22
2003	15.05
2004	14.93
2005	14.03
2006	13.61
2007	12.69
2008	12.54
2009	12.35
2010	12.24
2011	12.3
2012	12.22
2013	12.49
2014	13.15
2015	13.78
2016	14.29