Year	per 1,000
1990	10.3
1991	9.9
1992	9.5
1993	9.1
1994	8.7
1995	8.3
1996	8
1997	7.6
1998	7.3
1999	7
2000	6.7
2001	6.5
2002	6.9
2003	6.1
2004	6.3
2005	5.8
2006	5.6
2007	5.5
2008	5.4
2009	5.4
2010	5.3
2011	5.2
2012	5.2
2013	5.1
2014	5
2015	6.1
2016	4.8
2017	4.7
2018	4.6
2019	4.5
2020	4.4
2021	4.2
2022	4.1