Year	Number of
1990	1.89
1991	1.82
1992	1.75
1993	1.69
1994	1.62
1995	1.56
1996	1.57
1997	1.59
1998	1.61
1999	1.64
2000	1.66
2001	1.68
2002	1.7
2003	1.71
2004	1.71
2005	1.69
2006	1.67
2007	1.62
2008	1.58
2009	1.6
2010	1.63
2011	1.65
2012	1.69
2013	1.74
2014	1.79
2015	1.81
2016	1.84