Year	per 1,000
1990	2
1991	2
1992	2
1993	2
1994	2.1
1995	2.1
1996	2.1
1997	2
1998	2
1999	2
2000	1.9
2001	1.9
2002	1.8
2003	1.7
2004	1.7
2005	1.6
2006	1.5
2007	1.5
2008	1.4
2009	1.4
2010	1.3
2011	1.3
2012	1.2
2013	1.2
2014	1.1
2015	1.1
2016	1.1
2017	1.1
2018	1
2019	1
2020	1
2021	1
2022	1