Year	per 1,000
1990	5.1
1991	5.1
1992	5
1993	5
1994	4.9
1995	4.8
1996	4.6
1997	4.5
1998	4.3
1999	4.1
2000	3.9
2001	3.7
2002	3.5
2003	3.3
2004	3.1
2005	3
2006	2.9
2007	2.9
2008	2.9
2009	2.8
2010	2.8
2011	2.7
2012	2.7
2013	2.7
2014	2.7
2015	2.7
2016	2.7
2017	2.7
2018	2.6
2019	2.6
2020	2.5
2021	2.4
2022	2.3