Year	per 1,000
1990	1
1991	0.9
1992	0.9
1993	0.9
1994	0.8
1995	0.8
1996	0.8
1997	0.7
1998	0.7
1999	0.7
2000	0.6
2001	0.6
2002	0.6
2003	0.6
2004	0.5
2005	0.5
2006	0.5
2007	0.4
2008	0.4
2009	0.4
2010	0.4
2011	0.3
2012	0.3
2013	0.3
2014	0.3
2015	0.3
2016	0.3
2017	0.2
2018	0.2
2019	0.2
2020	0.2
2021	0.2
2022	0.2