Year	per 1,000
1990	7.4
1991	7.3
1992	7.1
1993	7
1994	6.8
1995	6.7
1996	6.6
1997	6.4
1998	6.3
1999	6.2
2000	6.1
2001	6
2002	6
2003	5.9
2004	5.9
2005	5.8
2006	5.7
2007	5.7
2008	5.6
2009	5.5
2010	5.4
2011	5.3
2012	5.2
2013	5.1
2014	5
2015	4.9
2016	4.8
2017	4.7
2018	4.6
2019	4.5
2020	4.4
2021	4.3
2022	4.2