Year	per 1,000
1990	2.8
1991	2.7
1992	2.6
1993	2.5
1994	2.5
1995	2.4
1996	2.4
1997	2.3
1998	2.3
1999	2.2
2000	2.2
2001	2.2
2002	2.1
2003	2.1
2004	2.1
2005	2
2006	2
2007	2
2008	2
2009	1.9
2010	1.9
2011	1.9
2012	1.9
2013	1.8
2014	1.8
2015	1.8
2016	1.7
2017	1.7
2018	1.7
2019	1.7
2020	1.6
2021	1.6
2022	1.5