Average lifetime cigarette use was assessed during the VETSA1 MRI study as cigarette pack-years, defined as average cigarette use over years of smoking. Pack-years was calculated for all participants as the number of cigarettes smoked per day multiplied by the number of years smoked. This product was divided by 20, or the number of cigarettes in a single pack. Current smokers were asked the number of cigarettes they usually smoke per day. Participants who were former smokers were asked the number of cigarettes they usually smoked per day during the last time they were a regular smoker. Both current and former smokers were asked the age at which they began smoking. Former smokers were asked the age at which they stopped smoking to determine the number of years smoked. Age at the time of data collection was used to determine smoking duration among current smokers. Individuals who indicated having smoked fewer than 100 cigarettes in their lifetime were classified as having an average lifetime cigarette use of zero pack-years. The distribution of pack-year values was skewed, so the data were
