Year	per 1,000
1990	3.1
1991	3
1992	2.9
1993	2.9
1994	2.8
1995	2.8
1996	2.7
1997	2.6
1998	2.6
1999	2.5
2000	2.3
2001	2.2
2002	2.1
2003	2
2004	1.9
2005	1.8
2006	1.7
2007	1.7
2008	1.6
2009	1.6
2010	1.6
2011	1.6
2012	1.5
2013	1.5
2014	1.5
2015	1.5
2016	1.4
2017	1.4
2018	1.3
2019	1.3
2020	1.3
2021	1.2
2022	1.2