Year	per 1,000
1990	6.2
1991	6.1
1992	6
1993	5.8
1994	5.6
1995	5.4
1996	5.1
1997	4.9
1998	4.6
1999	4.3
2000	4.1
2001	3.9
2002	3.7
2003	3.5
2004	3.4
2005	3.3
2006	3.2
2007	3.1
2008	3.1
2009	3
2010	3
2011	2.9
2012	2.9
2013	2.9
2014	2.8
2015	2.8
2016	2.8
2017	2.8
2018	2.8
2019	2.9
2020	2.9
2021	3.1
2022	3.2