Year	Number of
1990	24.19
1991	23.89
1992	23.06
1993	22.49
1994	22.04
1995	21.68
1996	21.6
1997	21.73
1998	21.13
1999	20.63
2000	20.34
2001	19.93
2002	19.24
2003	18.65
2004	18.56
2005	18.5
2006	18.34
2007	17.99
2008	18.09
2009	18.31
2010	18.31
2011	18.25
2012	18.41
2013	18.38
2014	18.35
2015	18.29
2016	17.82