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. Author manuscript; available in PMC: 2018 May 2.
Published in final edited form as: J Subst Abuse Treat. 2016 Dec 30;74:71–79. doi: 10.1016/j.jsat.2016.12.009

Table 2.

Treatment effect on drug use reductions, at end of intervention, 6 and 12 month follow-up (generalized linear model).

Overall treatment effectc
Contrasts
MI vs. Controld MI+HealthCall vs. Controld MI+HealthCall vs. MIe
χ2, df p Incidence risk ratio (95% confidence interval)
NumDUa
 End of intervention (at 60 days) (n = 224) 2.78, 2 0. 25 0.68 (0.43–1.08) 0.77 (0.49–1.21) 1.13 (0.71–1.78)
 6 month follow-up (n = 211) 7.27, 2 0.03 0.49 (0.29–0.85) 0.59 (0.35–1.00) 1.20 (0.70–2.06)
 12 month follow-up (n = 208) 4.88, 2 0.09 0.54 (0.31–0.94) 0.81 (0.47–1.38) 1.51 (0.87–2.62)
QuantUb
 End of intervention (at 60 days) (n = 224) 0.53, 2 0.77 0.83 (0.49–1.43) 0.85 (0.51–1.44) 1.02 (0.60–1.74)
 6 month follow-up (n = 211) 15.92, 2 <0.01 0.50 (0.28–0.90) 0.75 (0.42–1.33) 1.50 (0.84–2.67)
 12 month follow-up (n = 208) 15.08, 2 0.04 0.45 (0.25–0.83) 0.72 (0.40–1.31) 1.60 (0.87–2.93)

Incidence risk ratios of treatment group contrasts at study time points.

MI = motivational interviewing.

a

NumDU = mean number of days used primary drug during the 30 days prior to study visit.

b

QuantDU = mean dollar amount per day used over 30 days prior to study visit.

c

Corresponds to the omnibus test of treatment as a predictor of drug use outcome.

d

Control = Reference group.

e

MI = Reference group.