Table 1.

Generalized Estimating Equation (GEE) model of drug-positive hair tests.

	Any Drug OR (95% CI)	Marijuana OR (95% CI)	Cocaine OR (95% CI)	Amphetamines OR (95% CI)
Time (ref= baseline)
3 months	0.92 (0.67 - 1.27)	0.91 (0.67 - 1.24)	1.02 (0.79 - 1.32)	1.08 (0.61 - 1.91)
6 months	1.01 (0.70 - 1.45)	1.01 (0.73 - 1.41)	0.92 (0.67 - 1.28)	1.43 (0.80 - 2.55)
12 months	0.72 (0.49 - 1.04)	0.82 (0.58 - 1.15)	0.83 (0.57 - 1.20)	0.71 (0.33 - 1.55)
Condition (ref= IBI)
CBI	0.74 (0.47 - 1.18)	0.83 (0.53 - 1.29)	0.88 (0.49 - 1.60)	1.18 (0.52 - 2.66)
Condition × Time
3 months × CBI	1.12 (.74 – 1.67)	1.13 (0.76 - 1.67)	0.99 (0.69 - 1.43)	1.14 (0.55 - 2.37)
6 months × CBI	0.95 (0.61 - 1.49)	0.87 (0.57 - 1.33)	0.96 (0.63 - 1.45)	0.78 (0.36 - 1.70)
12 months × CBI	1.24 (0.75 - 2.07)	1.02 (0.63 - 1.66)	1.01 (0.61 - 1.70)	0.97 (0.34 - 2.75)
Overall Contrasts
Time Effect (overall change)	χ2(3)= 3.91; p= .27	χ2(3)= 2.74; p= .43	χ2(3)= 2.75; p= .43	χ2(3)= 6.25; p= .10
Condition × Time (differential change by condition)	χ2(3)= 1.73; p= .63	χ2(3)= 1.67; p= .65	χ2(3)= 0.07; p= .99	χ2(3)= 1.33; p= .72

Notes: OR= Odds Ratio; CI= Confidence Interval. Due to missing data, for any positive test: n= 334 (1,024 person-time observations); for marijuana n= 332 (1,001 person-time observations); for cocaine n= 334 (1,023 person-time observations); for amphetamines n= 334 (1,022 person-time observations). GEE models failed for opiates due to a very low number of positive tests (<4% at each time point).