Year	per 1,000
1990	3
1991	3
1992	3
1993	3
1994	3
1995	2.9
1996	2.9
1997	2.9
1998	2.8
1999	2.8
2000	2.7
2001	2.7
2002	2.6
2003	2.6
2004	2.6
2005	2.5
2006	2.5
2007	2.5
2008	2.4
2009	2.4
2010	2.4
2011	2.3
2012	2.3
2013	2.2
2014	2.2
2015	2.1
2016	2
2017	1.9
2018	1.9
2019	1.8
2020	1.7
2021	1.7
2022	1.6