Year	per 1,000
1990	6.9
1991	6.7
1992	6.6
1993	6.6
1994	6.5
1995	6.5
1996	6.4
1997	6.4
1998	6.4
1999	6.4
2000	6.4
2001	6.4
2002	6.4
2003	6.4
2004	6.3
2005	6.2
2006	6.1
2007	6.1
2008	6
2009	5.9
2010	5.8
2011	5.8
2012	5.7
2013	5.7
2014	5.7
2015	5.9
2016	6
2017	6.3
2018	6.6
2019	6.9
2020	7.3
2021	7.7
2022	8