Year	per 1,000
1990	7.1
1991	6.8
1992	6.5
1993	6.2
1994	5.9
1995	5.6
1996	5.4
1997	5.2
1998	5
1999	4.8
2000	4.7
2001	4.5
2002	4.4
2003	4.3
2004	4.3
2005	4.2
2006	4.2
2007	4.2
2008	4.2
2009	4.2
2010	4.1
2011	4.1
2012	4
2013	3.9
2014	3.8
2015	3.7
2016	3.6
2017	3.5
2018	3.4
2019	3.3
2020	3.2
2021	3.1
2022	3