Year	per 1,000
1990	5.7
1991	5.4
1992	5.2
1993	5.1
1994	5
1995	4.9
1996	4.8
1997	4.8
1998	4.9
1999	4.9
2000	5
2001	5.1
2002	5.2
2003	5.2
2004	5.3
2005	5.2
2006	5.2
2007	5.1
2008	4.9
2009	4.8
2010	4.6
2011	4.4
2012	4.2
2013	4.1
2014	3.9
2015	3.8
2016	3.7
2017	3.7
2018	3.6
2019	3.6
2020	3.6
2021	3.5
2022	3.5