Year	Number of
1990	17.67
1991	17.95
1992	17.8
1993	17.91
1994	17.96
1995	17.58
1996	17.23
1997	17.26
1998	17.04
1999	16.85
2000	16.91
2001	17.28
2002	17.15
2003	16.83
2004	16.66
2005	16.67
2006	16.8
2007	16.68
2008	16.35
2009	16.18
2010	16.87
2011	17.41
2012	17.62
2013	18.36
2014	19.03
2015	19.64
2016	19.98