Table 4.

IQ decline after holding education constant

Sample	Never used, never diagnosed	Used, never diagnosed	1 diagnosis	2 diagnoses	3+ diagnoses	Linear trend t test*	P
Full sample	0.05 (n = 242)	−0.07 (n = 479)	−0.11 (n = 80)	−0.17 (n = 35)	−0.38 (n = 38)	−4.45	<0.0001
High-school education or less	−0.03 (n = 59)	−0.14 (n = 130)	−0.16 (n = 43)	−0.25 (n = 20)	−0.48 (n = 26)	−3.36	0.0009

Mean change in full-scale IQ from childhood to adulthood is presented in SD units as a function of the number of study waves between ages 18 y and 38 y for which a study member met criteria for cannabis dependence. These change scores can be interpreted as effect sizes, with values of 0.20, 0.50, and 0.80 reflecting small, medium, and large effects, respectively. Change scores are presented for the full sample and for the sample of study members with a high-school education or less. Persistent cannabis dependence was associated with IQ decline in the full sample and the sample of study members with a high-school education or less.

*To test for a dose–response effect, we conducted an ordinary least-squares regression, estimating the linear trend controlling for sex.