Year	per 1,000
1990	4.5
1991	4.5
1992	4.4
1993	4.4
1994	4.3
1995	4.3
1996	4.2
1997	4.2
1998	4.1
1999	4.1
2000	4
2001	4
2002	3.9
2003	3.9
2004	3.8
2005	3.8
2006	3.7
2007	3.7
2008	3.6
2009	3.6
2010	3.6
2011	3.5
2012	3.5
2013	3.4
2014	3.4
2015	3.4
2016	3.3
2017	3.3
2018	3.2
2019	3.2
2020	3.2
2021	3.1
2022	3.1