Using and citing the calculator in research

How does the method work?

For each period entered, the amount is converted into cigarettes, weekly amounts are divided by 7, and cigarettes a day are multiplied by years. The periods are added together, divided by 20 and rounded to the nearest whole number. The factors, units and a worked example are on how the calculation works.

The factors are the authors’ Reference Standards 2010. Those for cigars, cigarillos, pipes and loose tobacco are conversion figures taught at the National Respiratory Training Centre at Warwick, now Education for Health. For spliffs, waterpipes and bidis (added in 2024), the References page cites Aldington et al (Thorax 2007), the WHO TobReg advisory note (2005) and “Concept of BIDI years” (Lung India 2022).

What are its limitations?

• The References page says: “This is not an exact science and approximations have been made.” The original work behind the Warwick figures “could not be found by the present authors”; they were kept because large numbers of practice nurses had been trained using them.
• Wood et al (Tobacco Control 2005) and the 2003 Australian Gulf War veterans’ health study were used to check the figures against others working in this area.
• The result relies on an accurate history (FAQ 8).
• FAQ 2 notes that 5 cigarettes a day for 20 years and 20 a day for 5 years are both 5 pack years, though the risk of smoking-related ill health is very much greater in the first case. It calls the measure “only a crude imprecise description of smoking related exposure”.
• Vaping and passive smoking are not counted (FAQs 9 and 11).

FAQ 2 recommends recording starting and stopping dates and years smoked alongside pack years; the optional history step asks for the date stopped and years smoked. FAQ 2 says numerical smoking records would help “epidemiological studies and planning health care interventions”.

How should I cite the calculator?

The suggested citation is:

Masters NJ, Tutt C. Smoking pack years calculator, version 2.0 [online]. Available at: https://www.smokingpackyears.com/ (accessed date).

The record line and the printed result name the method as “Masters and Tutt, Reference Standards 2010 (bidis added 2024)” and give the calculator version. The calculator is also described in Masters NJ. Smoking pack years calculator. Br J Gen Pract. 2020;70(694):230. doi:10.3399/bjgp20X709553. Dr Masters is its sole author. Other papers are listed on publications and awards.

How has the calculator changed?