Year	per 1,000
1990	9
1991	8.5
1992	8
1993	7.6
1994	7.2
1995	6.8
1996	6.6
1997	6.5
1998	6.4
1999	6.4
2000	6.4
2001	6.5
2002	6.5
2003	6.5
2004	6.5
2005	6.5
2006	6.5
2007	6.5
2008	6.5
2009	6.5
2010	6.4
2011	6.3
2012	6.1
2013	6.1
2014	6
2015	5.9
2016	5.8
2017	5.7
2018	5.6
2019	5.5
2020	5.6
2021	5.7
2022	5.8