Year	per 1,000
1990	5
1991	5
1992	4.9
1993	4.9
1994	4.8
1995	4.8
1996	4.7
1997	4.7
1998	4.6
1999	4.6
2000	4.5
2001	4.5
2002	4.5
2003	4.4
2004	4.4
2005	4.3
2006	4.3
2007	4.2
2008	4.2
2009	4.1
2010	4.1
2011	4.1
2012	4
2013	4
2014	3.9
2015	3.9
2016	3.9
2017	3.8
2018	3.8
2019	3.7
2020	3.7
2021	3.6
2022	3.6