Year	per 1,000
1990	4.7
1991	4.7
1992	4.6
1993	4.6
1994	4.6
1995	4.7
1996	4.6
1997	4.6
1998	4.6
1999	4.5
2000	4.5
2001	4.5
2002	4.4
2003	4.3
2004	4.5
2005	4.1
2006	4.1
2007	4
2008	4
2009	3.8
2010	3.7
2011	3.6
2012	3.6
2013	3.5
2014	3.5
2015	3.5
2016	3.5
2017	3.5
2018	3.5
2019	3.5
2020	3.5
2021	3.4
2022	3.4