Year	per 1,000
1990	27
1991	26.6
1992	26.3
1993	25.9
1994	25.6
1995	25.2
1996	29.6
1997	24.6
1998	24.2
1999	23.9
2000	23.6
2001	23.3
2002	23
2003	22.7
2004	22.4
2005	22.1
2006	21.8
2007	21.6
2008	21.5
2009	21.3
2010	21.2
2011	21
2012	20.9
2013	20.8
2014	20.7
2015	20.5
2016	20.4
2017	20.2
2018	20
2019	19.8
2020	19.6
2021	19.4
2022	19.3