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	6.8
1998	8.2
1999	6.4
2000	6.3
2001	6.1
2002	5.9
2003	5.8
2004	5.6
2005	5.5
2006	5.3
2007	5.2
2008	5.1
2009	5
2010	4.9
2011	4.8
2012	4.7
2013	4.6
2014	4.5
2015	4.4
2016	4.3
2017	4.2
2018	4.2
2019	4.1
2020	4
2021	3.9
2022	3.9