Duration of exposure to risk was set to the number of months between the Wave 1 and Wave 2 interviews minus the number of months since last drink. For individuals who did not start drinking until after the Wave 1 interview, this duration was further reduced by the number of months between the Wave 1 interview and the first drink, assumed to be 6 months if age at Wave 2 interview was equal to age at first drink, 12 months if they were one year apart, etc.
