Our study is not without limitations. One important limitation is that we have not confirmed that the substance measures predated the educational attainment. We can be almost certain that early use (and probably daily nicotine use) occurred before participants finished their schooling, however there is less certainty regarding the temporal sequence for the dependence measures. Although we can date the onset of first dependence symptom, we do not have any measures that assess when the individuals completed their formal education, making such an examination impossible with the present data.
