Year	Number of
1990	14.87
1991	15.23
1992	15.56
1993	15.9
1994	16.09
1995	15.99
1996	15.7
1997	15.47
1998	15.12
1999	14.79
2000	14.68
2001	14.83
2002	15.51
2003	16.52
2004	16.4
2005	15.85
2006	15.55
2007	15.74
2008	16.35
2009	16.96
2010	17.77
2011	17.87
2012	18.04
2013	18.22
2014	18.65
2015	19.2
2016	19.59