Sibling relationships were measured by offspring reports on how much they could discuss worries with any sibling, how much they thought any sibling would help with problems and how much they thought a sibling would understand them. Responses to each question were on a 4 level scale ranging from `a lot' to `not at all'. Because the sibling support variables were highly correlated, we created a 10 level ordinal sibling support variable by summing the responses to each 4 point scale which produced a range of values from 3 to 12 that we re-scaled to values 1 to 10. The chronbach alpha for this measure was 0.86 and we have used this scaling procedure for previous studies of sibling support and cannabis/abuse dependence (Scherrer et al. 2008).
