Table 1.

The ED50 values and 95% Confidence Limits (95% C.L.) for the past five years, calculated twice per year for the same group of rats, trained to discriminate (+)-amphetamine (1 mg/kg; 5.4 μmol/kg, i.p.) from saline. N = 8–16 rats per group. “a” – the ED50 calculated from tests immediately after rats passed the criterion for accuracy (see details in Methods section), and “b” –Calculated based on results from tests run 6–8 months later for the same group of rats. The ED50 was calculated according to the method of Litchfield and Wilcoxon (1949).

Year	ED50	95% C.L.	ED50	95% C.L.
	μmol/kg		mg/kg
2004a	1.31	0.88–1.97	0.24	0.16–0.37
2004b	1.34	0.93–1.95	0.24	0.17–0.36
2005a	1.55	1.04–2.31	0.29	0.19–0.46
2005b	1.60	1.03–2.20	0.30	0.19–0.46
2006a	1.43	0.93–2.21	0.26	0.17–0.41
2006b	1.77	1.42–2.22	0.33	0.26–0.41
2007a	1.23	0.81–1.85	0.23	0.15–0.34
2007b	1.25	0.84–1.87	0.23	0.16–0.35
2008a	1.31	0.89–1.22	0.24	0.17–0.34
2008b	1.45	0.89–2.38	0.27	0.16–0.44