Year	per 1,000
1990	8.7
1991	8.3
1992	8
1993	7.7
1994	7.3
1995	6.9
1996	6.5
1997	6.1
1998	5.7
1999	5.4
2000	5.1
2001	4.8
2002	4.6
2003	4.4
2004	4.2
2005	4
2006	3.8
2007	3.6
2008	3.4
2009	3.3
2010	3.1
2011	3
2012	2.9
2013	2.8
2014	2.7
2015	2.7
2016	2.6
2017	2.5
2018	2.5
2019	2.4
2020	2.3
2021	2.2
2022	2.2