Year	per 1,000
1990	3.2
1991	3
1992	2.9
1993	2.8
1994	2.8
1995	2.8
1996	2.9
1997	2.9
1998	2.9
1999	2.8
2000	2.6
2001	2.4
2002	2.3
2003	2.1
2004	2
2005	2
2006	1.9
2007	1.8
2008	1.8
2009	1.7
2010	1.7
2011	1.6
2012	1.6
2013	1.5
2014	1.5
2015	1.5
2016	1.5
2017	1.5
2018	1.5
2019	1.5
2020	1.5
2021	1.5
2022	1.5