Year	per 1,000
1990	3.5
1991	3.7
1992	3.7
1993	3.6
1994	3.5
1995	6.7
1996	6.7
1997	6.6
1998	6.6
1999	6.5
2000	6.5
2001	6.4
2002	6.3
2003	3
2004	2.9
2005	2.9
2006	2.9
2007	2.9
2008	2.8
2009	2.8
2010	2.7
2011	2.5
2012	2.4
2013	2.3
2014	2.2
2015	2.1
2016	2
2017	1.9
2018	1.9
2019	1.8
2020	1.7
2021	1.7
2022	1.8