Year	Number of
1990	18.22
1991	18.38
1992	18.55
1993	18.05
1994	17.48
1995	16.94
1996	16.81
1997	16.85
1998	17.21
1999	17.77
2000	18.24
2001	18.51
2002	18.59
2003	18.74
2004	18.93
2005	19.08
2006	18.71
2007	18.52
2008	18.19
2009	17.82
2010	17.75
2011	18.19
2012	18.55
2013	18.13
2014	17.95
2015	18.07
2016	18.4