Year	per 1,000
1990	8
1991	7.9
1992	7.7
1993	7.5
1994	7.3
1995	7.1
1996	6.9
1997	6.8
1998	6.6
1999	6.4
2000	6.2
2001	6
2002	5.8
2003	5.6
2004	5.4
2005	5.3
2006	5.1
2007	5
2008	4.8
2009	4.7
2010	4.6
2011	4.4
2012	4.3
2013	4.2
2014	4.1
2015	4.1
2016	4
2017	3.9
2018	3.8
2019	3.7
2020	3.6
2021	3.6
2022	3.5