Year	per 1,000
1990	27.3
1991	22.7
1992	22.6
1993	22.4
1994	22.3
1995	22.2
1996	22
1997	21.8
1998	21.6
1999	21.3
2000	21.1
2001	20.7
2002	20.4
2003	20.1
2004	19.8
2005	19.5
2006	19.2
2007	18.9
2008	18.8
2009	18.6
2010	18.4
2011	18.3
2012	18.1
2013	18
2014	17.7
2015	17.4
2016	17.1
2017	16.6
2018	16.2
2019	15.8
2020	15.3
2021	15
2022	14.5