Year	per 1,000
1990	4.8
1991	4.7
1992	4.6
1993	4.5
1994	4.4
1995	4.3
1996	4.3
1997	4.2
1998	4.2
1999	4.1
2000	4.1
2001	4.1
2002	4
2003	4
2004	4
2005	4
2006	4
2007	3.9
2008	3.9
2009	3.9
2010	3.9
2011	3.9
2012	3.9
2013	3.9
2014	3.9
2015	3.9
2016	3.9
2017	3.8
2018	3.8
2019	3.7
2020	3.7
2021	3.6
2022	3.6