Year	per 1,000
1990	5
1991	5
1992	4.9
1993	4.8
1994	4.8
1995	4.8
1996	4.7
1997	4.5
1998	4.4
1999	4.2
2000	4.1
2001	4
2002	3.9
2003	3.8
2004	3.7
2005	3.6
2006	3.6
2007	3.6
2008	3.7
2009	3.7
2010	3.6
2011	3.4
2012	3.3
2013	3.2
2014	3.2
2015	3.1
2016	3.2
2017	3.2
2018	3.2
2019	3.3
2020	3.3
2021	3.4
2022	3.4