Year	per 1,000
1990	5.3
1991	5.2
1992	5
1993	4.8
1994	4.7
1995	4.5
1996	4.4
1997	4.2
1998	4
1999	3.9
2000	3.7
2001	3.6
2002	3.5
2003	3.4
2004	3.3
2005	3.2
2006	3.1
2007	3
2008	3
2009	2.9
2010	2.8
2011	2.8
2012	2.7
2013	2.6
2014	2.6
2015	2.5
2016	2.4
2017	2.4
2018	2.3
2019	2.2
2020	2.2
2021	2.1
2022	2.1