Year	Percent (Units)
1980	21.305
1981	21.393
1982	20.926
1983	21.370
1984	21.946
1985	22.070
1986	22.071
1987	22.001
1988	21.926
1989	21.926
1990	21.473
1991	20.938
1992	19.512
1993	19.665
1994	19.870
1995	19.681
1996	19.663
1997	19.738
1998	20.064
1999	20.318
2000	20.180
2001	19.887
2002	19.675
2003	19.436
2004	19.170
2005	18.950
2006	18.508
2007	17.928
2008	17.438
2009	17.063
2010	16.650
2011	16.276
2012	16.105
2013	15.939
2014	16.025
2015	16.309
2016	16.142
2017	15.984
2018	15.892
2019	15.846
2020	15.957
2021	15.867
2022	15.648
2023	15.563
2024	15.501
2025	15.302
2026	15.138
2027	14.993
2028	14.857
2029	14.723