Year	per 1,000
1990	8.1
1991	7.8
1992	9.3
1993	7.5
1994	7.2
1995	6.9
1996	6.6
1997	6.3
1998	6
1999	5.8
2000	5.5
2001	5.2
2002	5
2003	4.8
2004	4.7
2005	4.5
2006	4.4
2007	4.4
2008	4.3
2009	4.2
2010	4.1
2011	4
2012	4
2013	3.9
2014	3.9
2015	4
2016	4
2017	4
2018	4
2019	4.1
2020	4.1
2021	4.1
2022	4.1