Year	per 1,000
1990	5.2
1991	5.1
1992	5.1
1993	5
1994	4.9
1995	4.8
1996	4.6
1997	4.5
1998	4.4
1999	4.2
2000	4.1
2001	4
2002	3.9
2003	3.7
2004	3.6
2005	3.5
2006	3.3
2007	3.2
2008	3
2009	2.8
2010	2.7
2011	2.6
2012	2.4
2013	2.3
2014	2.2
2015	2.1
2016	2
2017	1.9
2018	1.8
2019	1.7
2020	1.6
2021	1.5
2022	1.4