Data were prepared for raw ordinal data analysis in the software Mx (33). This approach assumes that the observed ordinal categories within each item are an imprecise measure of a latent normal distribution of liability, and that this liability distribution has one or more threshold values which discriminate between the categories. Thresholds can be conceived of as cut points along a standard normal distribution which classify individuals in terms of a probability or risk of endorsing one of two or more discrete (ordinal) categories. The cannabis availability were summed across the five epochs and recoded onto a five point ordinal scale ranging from 0 to 4. A total cannabis abuse score based on the six diagnostic criterion was also calculated. This was recoded onto a 4 ordinal scale ranging from 0 to 3. All thresholds were adjusted for the linear effects of age.
