Dransfield and colleagues1 reported, using an observational study design, that inpatient use of β blockers by patients hospitalised for COPD is associated with a surprisingly important 61% reduction in mortality.1 They also reported an astonishing 92% reduction in mortality associated with short acting β agonist use. Several biases introduced in the design and analysis of this observational study must be considered when interpreting these startling mortality reductions in patients with COPD.

Firstly, immortal time bias was introduced by defining exposure to β blockers or to short acting β agonists by billings occurring at any time during the hospitalisation.2 Indeed, the exposed patients necessarily had some initial period with no exposure before they received these drugs during the course of their hospitalisation. This …