Year	per 1,000
1990	5
1991	4.8
1992	4.6
1993	4.5
1994	4.3
1995	4.1
1996	3.9
1997	3.7
1998	3.5
1999	3.4
2000	3.2
2001	3
2002	2.9
2003	2.8
2004	2.6
2005	2.5
2006	2.3
2007	2.2
2008	2.1
2009	2
2010	1.9
2011	1.8
2012	1.7
2013	1.7
2014	1.6
2015	1.6
2016	1.5
2017	1.5
2018	1.4
2019	1.4
2020	1.3
2021	1.3
2022	1.3