Year	per 1,000
1990	4
1991	4
1992	3.9
1993	3.9
1994	3.8
1995	3.8
1996	3.7
1997	3.6
1998	3.5
1999	3.5
2000	3.4
2001	3.3
2002	3.2
2003	3.2
2004	3.1
2005	3
2006	3
2007	2.9
2008	2.9
2009	2.8
2010	2.8
2011	2.7
2012	2.7
2013	2.7
2014	2.6
2015	2.6
2016	2.5
2017	2.5
2018	2.5
2019	2.4
2020	2.4
2021	2.3
2022	2.3