Year	per 1,000
1990	6.1
1991	5.8
1992	5.5
1993	5.3
1994	5
1995	4.8
1996	4.6
1997	4.4
1998	4.2
1999	4
2000	3.9
2001	3.7
2002	3.6
2003	3.5
2004	3.5
2005	3.4
2006	3.4
2007	3.3
2008	3.3
2009	3.3
2010	3.2
2011	3.2
2012	3.2
2013	3.1
2014	3.1
2015	3
2016	2.9
2017	2.9
2018	2.8
2019	2.7
2020	2.6
2021	2.5
2022	2.4