What level of significance did they calculate that these results were at? Were there roughly equal numbers of subjects considered in each IQ sextile, or were they working from a sample that had a normal distribution of IQ?
It seems that with this sample size, if it was a normally distributed sample, you'd probably only have about 106 people at 98th percentile and above (over 130 IQ). That is few enough that each individual who reported in the affirmative would account for nearly a full percentage point increase in the rate of drug use for that group, and that could make a huge difference in the reported relative chances, since, IIRC, the rates of drug use they found at age 30 were in the single digits.
