Year	per 1,000
1990	4.7
1991	4.5
1992	4.3
1993	4.2
1994	4
1995	3.8
1996	3.7
1997	3.6
1998	3.5
1999	3.4
2000	3.3
2001	3.2
2002	3.1
2003	3
2004	3
2005	2.9
2006	2.9
2007	2.9
2008	2.9
2009	2.9
2010	2.8
2011	2.8
2012	2.8
2013	2.8
2014	2.8
2015	2.8
2016	2.8
2017	2.8
2018	2.7
2019	2.7
2020	2.7
2021	2.7
2022	2.6