Year	Percent change (Units)
1980	14.236
1981	15.348
1982	13.721
1983	12.797
1984	11.345
1985	16.594
1986	18.102
1987	16.490
1988	12.795
1989	14.642
1990	14.386
1991	15.153
1992	14.012
1993	9.720
1994	8.773
1995	8.849
1996	7.338
1997	8.613
1998	6.973
1999	5.134
2000	5.322
2001	5.705
2002	9.044
2003	5.921
2004	1.387
2005	3.367
2006	4.662
2007	7.139
2008	11.020
2009	7.179
2010	4.211
2011	4.993
2012	5.648
2013	5.742
2014	6.104
2015	4.588
2016	6.333
2017	5.279
2018	4.612
2019	4.121
2020	3.275
2021	4.556
2022	6.869
2023	5.900
2024	4.863
2025	4.500
2026	4.500
2027	4.500
2028	4.500
2029	4.500