who used cannabis later, two series of conditional logistic regression analyses were conducted: one examining both later-using and never-using cotwins (n=293), and one restricted to the subset of discordant pairs in which the cotwin initiated cannabis use later (n=190). These cotwin-control analyses permit the assessment of whether any associations are: a) attributable to an underlying familial liability, in which case one would expect early- and later/never- using twin members would be at equal risk for later problems, b) are causal (or due to a third unmeasured environmental risk factor), in which case the early-using twin would remain at greater risk for problems even after controlling for familial influences, or c) are influenced by both (a) and (b).
