Table 2.

Repeated measures of marijuana use at 6- and 12-month follow-up among participants with any marijuana use at baseline

Measure	MI (n = 55)	FO(n = 41)	Post hoc tests within 6-month timeframe			Effect Size
	Mean (SD)	Mean (SD)	df	F (time)	F (Time × Tx)
No. days used marijuana past 30 days
Baseline	15.83 (11.60)	11.78 (11.37)
6-month follow-up	12.24 (11.75)	8.71 (10.88)	1/94	8.46**	0.05	.31 (MI)
						.28 (FO)
12-month follow-up	9.37 (10.97)	9.91 (11.59)	1/94	0.80	4.72*	.57(MI)
						.16 (FO)
	MI (n = 25)	FO (n = 33)
	Mean (SD)	Mean (SD)
No. days used marijuana with alcohol past 30 days
Baseline	6.48 (6.58)	4.15 (4.77)
6-month follow-up, with 6-month alcohol use controlled	2.92 (3.40)	6.36 (6.98)	1/56	0.65	11.92‡	.54 (MI)
			1/56	12.43†	2.07	−.46 (FO)
12-month follow-up	3.13 (4.23)	4.29 (5.82)	1/56	0.19	0.67	.51 (MI)
						−.03 (FO)

Notes: Sample size for analyses are participants for which follow-up data were available (attrition 19% at 12 months with no significant differences by treatment condition; Monti et al., 2007). MI = motivational interviewing with personalized feedback; FO = personalized feedback only; Tx = treatment.

^* p < .05

^† p < .01

^** p < .005

^‡ p < .001.