Year	Percent (Units)
1980	3.803
1981	3.699
1982	3.757
1983	3.821
1984	3.741
1985	3.759
1986	3.746
1987	3.805
1988	3.836
1989	3.789
1990	3.663
1991	3.525
1992	3.181
1993	3.190
1994	3.204
1995	3.165
1996	3.126
1997	3.152
1998	3.171
1999	3.159
2000	3.144
2001	3.149
2002	3.118
2003	3.090
2004	3.006
2005	2.950
2006	2.870
2007	2.797
2008	2.712
2009	2.598
2010	2.524
2011	2.457
2012	2.420
2013	2.419
2014	2.427
2015	2.467
2016	2.482
2017	2.488
2018	2.436
2019	2.410
2020	2.224
2021	2.272
2022	2.294
2023	2.228
2024	2.170
2025	2.135
2026	2.106
2027	2.078
2028	2.049
2029	2.016