Year	per 1,000
1990	2.5
1991	2.6
1992	2.6
1993	2.6
1994	2.6
1995	2.5
1996	2.5
1997	2.5
1998	2.5
1999	2.4
2000	2.4
2001	2.4
2002	2.3
2003	2.3
2004	2.2
2005	2.2
2006	2.1
2007	2
2008	2
2009	1.9
2010	1.8
2011	1.8
2012	1.7
2013	1.7
2014	1.6
2015	1.6
2016	1.6
2017	1.5
2018	1.5
2019	1.5
2020	1.5
2021	1.5
2022	1.5