Year	per 1,000
1990	4.6
1991	4.4
1992	4.3
1993	4.4
1994	4.6
1995	4.8
1996	4.8
1997	4.7
1998	4.4
1999	4.2
2000	4
2001	4
2002	3.9
2003	3.8
2004	3.7
2005	3.5
2006	3.4
2007	3.3
2008	3.3
2009	3.4
2010	3.5
2011	3.5
2012	3.5
2013	3.4
2014	3.4
2015	3.4
2016	3.4
2017	3.3
2018	3
2019	2.6
2020	2.3
2021	2.2
2022	2.1