Year	per 1,000
1990	8.3
1991	8.1
1992	7.9
1993	7.7
1994	7.5
1995	7.3
1996	7.1
1997	6.9
1998	6.8
1999	6.6
2000	6.5
2001	6.4
2002	6.3
2003	6.2
2004	6.2
2005	6.1
2006	6.1
2007	6.1
2008	6.1
2009	6.1
2010	6
2011	6
2012	5.9
2013	5.8
2014	5.7
2015	5.6
2016	5.6
2017	5.5
2018	5.4
2019	5.3
2020	5.2
2021	5.1
2022	5