Year	per 1,000
1990	5
1991	4.9
1992	4.8
1993	4.7
1994	4.6
1995	4.5
1996	4.4
1997	4.3
1998	4.2
1999	4.1
2000	3.9
2001	3.8
2002	3.7
2003	3.6
2004	3.5
2005	3.4
2006	3.3
2007	3.2
2008	3.1
2009	3.1
2010	3
2011	2.9
2012	2.9
2013	2.8
2014	2.7
2015	2.7
2016	2.6
2017	2.6
2018	2.5
2019	2.5
2020	2.4
2021	2.4
2022	2.4