The limits of single-group interrupted time series analysis in assessing the impact of smoke-free laws on short-term mortality

Background. Decreases in circulatory/respiratory morbimortality after the January-2006 Spanish partial smoke-free law have been found using designs without control groups, such as single-group interrupted time series (ITS), which are prone to biases. The aim was to reassess the law's impact on mortality using ITS designs with robustness checks. Methods: A comprehensive cohort of people aged >= 25 in each calendar-year of 2002-2007, living in 13 of 18 Spanish regions, was followed up between 01/2002 and 12/2007. The law included a smoking ban in indoor public and workplaces, allowing exceptions in catering, hospitality and leisure venues, and other interventions. Post-law changes in monthly coronary/respiratory mortality were estimated using segmented regression, adjusting for relevant covariates, including seasonality, extreme temperatures, influenza incidence and air pollution. The validity of results was assessed using control outcomes, hypothetical law dates, and non-equivalent control groups, analysing their results as difference-in-differences (DID) designs. Results: Significant immediate post-law decreases in coronary, respiratory and non-tobacco-related mortality were observed among people aged >= 70. A significant immediate post-law decrease in respiratory mortality (-12.7%) was also observed among people age 25-69, although this was neutralized by a subsequent upward trend before 1.5 years. More favourable post-law changes in coronary/respiratory mortality among the target (people aged 25-69) than control groups (people aged >= 70 or women aged >= 80) were not identified in DID designs. Establishing hypothetical law dates, immediate decreases began in February/March 2005 with maxima between April and July 2005. Conclusions: After robustness checks, the results do not support a clear positive impact of the 2006 Spanish smoke-free law on short-term coronary/respiratory mortality. The favourable immediate changes observed pre- and post-law could derive mainly from the harvesting effect of the January-2005 cold wave. This highlights the risks of assessing the impact of health interventions using both morbimortality outcomes and designs without a control group and adequate robustness checks.