Year	per 1,000
1990	31
1991	17.4
1992	17.8
1993	18.1
1994	18.5
1995	18.8
1996	19.1
1997	19.4
1998	19.6
1999	19.8
2000	20
2001	20.2
2002	20.3
2003	20.4
2004	20.5
2005	20.5
2006	20.6
2007	20.6
2008	20.6
2009	20.6
2010	20.6
2011	20.5
2012	20.4
2013	20.3
2014	20.3
2015	20.2
2016	20.1
2017	19.9
2018	19.7
2019	19.4
2020	19.2
2021	19
2022	18.8