Year	per 1,000
1990	5.4
1991	5.2
1992	5.1
1993	5
1994	4.9
1995	4.8
1996	4.7
1997	4.5
1998	4.4
1999	4.2
2000	4.1
2001	4
2002	3.8
2003	3.7
2004	3.8
2005	3.4
2006	3.2
2007	3.1
2008	3.2
2009	3
2010	2.9
2011	2.9
2012	2.8
2013	2.8
2014	2.7
2015	2.7
2016	2.6
2017	2.6
2018	2.5
2019	2.5
2020	2.4
2021	2.4
2022	2.4