Year	per 1,000
1990	6.4
1991	6.1
1992	5.8
1993	5.6
1994	5.4
1995	5.2
1996	4.9
1997	4.7
1998	4.5
1999	4.2
2000	4
2001	3.9
2002	3.8
2003	3.7
2004	3.6
2005	3.6
2006	3.5
2007	3.5
2008	3.4
2009	3.4
2010	3.3
2011	3.2
2012	3
2013	2.9
2014	2.8
2015	2.8
2016	2.8
2017	2.8
2018	2.8
2019	2.8
2020	2.7
2021	2.7
2022	2.7