Year	per 1,000
1990	17.5
1991	17.8
1992	18.1
1993	18.4
1994	18.7
1995	19
1996	19.3
1997	19.6
1998	19.8
1999	20
2000	20.2
2001	20.2
2002	20.2
2003	20.2
2004	20.1
2005	20.1
2006	19.9
2007	19.8
2008	19.6
2009	19.4
2010	19.2
2011	19
2012	18.8
2013	18.6
2014	18.5
2015	18.4
2016	18.2
2017	18.1
2018	17.9
2019	17.8
2020	17.6
2021	17.5
2022	17.3