Year	Percent change (Units)
1981	13.158
1982	11.628
1983	12.500
1984	11.111
1985	20.000
1986	16.667
1987	15.476
1988	12.371
1989	15.596
1990	15.079
1991	15.862
1992	9.524
1993	9.783
1994	9.406
1995	7.240
1996	9.283
1997	6.178
1998	9.091
1999	2.333
2000	6.515
2001	4.893
2002	12.245
2003	0.260
2004	3.627
2005	3.500
2006	6.039
2007	8.656
2008	10.066
2009	6.310
2010	3.597
2011	6.076
2012	5.728
2013	5.263
2014	5.441
2015	5.160
2016	6.764
2017	4.695
2018	4.921
2019	3.744
2020	3.172
2021	5.442
2022	7.406
2023	5.523
2024	4.500
2025	4.500
2026	4.500
2027	4.500
2028	4.500
2029	4.500