Year	per 1,000
1990	3.7
1991	3.6
1992	3.6
1993	3.6
1994	3.7
1995	3.7
1996	3.8
1997	3.8
1998	3.9
1999	3.9
2000	3.9
2001	3.9
2002	3.8
2003	3.7
2004	3.6
2005	3.5
2006	3.6
2007	3.6
2008	3.7
2009	3.8
2010	3.9
2011	4
2012	4
2013	4
2014	3.9
2015	3.8
2016	3.6
2017	3.5
2018	3.3
2019	3.2
2020	3
2021	2.9
2022	2.8