Year	per 1,000
1990	5.2
1991	5
1992	4.8
1993	4.7
1994	4.5
1995	4.4
1996	4.2
1997	4.1
1998	4
1999	3.9
2000	3.8
2001	3.7
2002	3.6
2003	3.6
2004	3.5
2005	3.5
2006	3.4
2007	3.4
2008	3.4
2009	3.4
2010	3.4
2011	3.4
2012	3.3
2013	3.3
2014	3.3
2015	3.3
2016	3.3
2017	3.3
2018	3.3
2019	3.3
2020	3.2
2021	3.2
2022	3.2