Is calculating pack-years retrospectively a valid method to estimate life-time tobacco smoking? A comparison between prospectively calculated pack-years and retrospectively calculated pack-years

Aims. To investigate the relative validity of retrospectively calculated pack‐years (py‐retro) by comparing py‐retro with prospectively calculated pack‐years (py‐pro). Design. A 23‐year ongoing cohort study (1977‐2000). Participants. One hundred and fifty‐four males and females, 13 years old in 1977 and 36 years old in 2000. Setting. Amsterdam, the Netherlands. Measurements. To calculate py‐pro, current smoking and quitting efforts were investigated nine times in a period of 23 years with the help of an interview or a questionnaire. At the age of 36, subjects filled out a comprehensive questionnaire about their smoking history, to calculate py‐retro. Individual differences between py‐pro and py‐retro were calculated. In addition, Cohen's kappa was calculated after categorising py‐pro and py‐retro into three groups. Findings. (1) Py‐retro does not under‐ or overestimate life‐time tobacco smoking. (2) The relative validity of py‐retro was moderate due to large individual differences between py‐pro and py‐retro. (3) The individual differences between py‐pro and py‐retro became larger, the higher the number of pack‐years. (4) Mean difference (and 95% limits of agreement) between py‐pro and py‐retro was ‐0.039 (‐5.23, 5.32) when average pack‐years was