Year	per 1,000
1990	5.7
1991	6
1992	6.2
1993	6.2
1994	6.1
1995	5.9
1996	5.6
1997	5.3
1998	5
1999	4.7
2000	4.4
2001	4.2
2002	4
2003	3.8
2004	3.6
2005	3.4
2006	3.3
2007	3.2
2008	3
2009	2.9
2010	2.7
2011	2.6
2012	2.5
2013	2.5
2014	2.5
2015	2.4
2016	2.4
2017	2.4
2018	2.4
2019	2.3
2020	2.2
2021	2.1
2022	2