Year	per 1,000
1990	24
1991	23.9
1992	23.7
1993	23.4
1994	23.2
1995	25.2
1996	22.6
1997	22.3
1998	22
1999	21.8
2000	21.8
2001	21.8
2002	21.9
2003	22
2004	22.2
2005	22.3
2006	22.3
2007	22.1
2008	21.9
2009	21.6
2010	21.3
2011	21
2012	20.7
2013	20.4
2014	20
2015	19.6
2016	19.2
2017	18.7
2018	18.2
2019	17.7
2020	17.2
2021	16.7
2022	16.2