Year	per 1,000
1990	6.1
1991	5.1
1992	4.8
1993	4.5
1994	4.3
1995	4.1
1996	4
1997	3.8
1998	3.6
1999	3.5
2000	3.4
2001	3.2
2002	3.1
2003	3.4
2004	2.9
2005	2.9
2006	2.8
2007	2.8
2008	2.8
2009	2.7
2010	2.7
2011	2.8
2012	3.1
2013	3.2
2014	3
2015	2.9
2016	2.6
2017	2.6
2018	2.6
2019	2.3
2020	2.1
2021	2
2022	1.9