Year	Number of
1990	19.72
1991	19.98
1992	20.25
1993	20.46
1994	20.65
1995	20.78
1996	20.86
1997	20.8
1998	20.73
1999	20.77
2000	20.86
2001	21.18
2002	21.58
2003	21.77
2004	21.82
2005	21.84
2006	22.03
2007	22.08
2008	22.34
2009	22.64
2010	22.97
2011	23.67
2012	24.28
2013	25
2014	25.6
2015	26.81
2016	28.35