Year	per 1,000
1990	1
1991	1
1992	0.9
1993	0.9
1994	0.9
1995	0.9
1996	0.8
1997	0.8
1998	0.8
1999	0.8
2000	0.8
2001	0.7
2002	0.7
2003	0.7
2004	0.7
2005	0.7
2006	0.6
2007	0.6
2008	0.6
2009	0.6
2010	0.6
2011	0.5
2012	0.5
2013	0.5
2014	0.5
2015	0.5
2016	0.5
2017	0.4
2018	0.4
2019	0.4
2020	0.4
2021	0.4
2022	0.4