Bradford–Hill criteria for causality applied to the association between COPD, asthma and bronchiectasis
| Bradford Hill criteria | COPD | Asthma |
| Strength of association/effect size | Some studies report strong associations but may have methodological limitations | Strong association demonstrated |
| Consistency/reproducibility | Reported by several studies with most reporting a high prevalence | Reported by several studies with most reporting a high prevalence |
| Specificity | Lacking, as there are often other possible causes present | Lacking, as there are often other possible causes present |
| Temporality | Rarely demonstrated | Rarely demonstrated |
| Biological gradient | Clearly demonstrated across multiple studies that as COPD becomes more severe bronchiectasis is more prevalent | Some evidence that bronchiectasis is more common in more severe asthma |
| Plausibility | Both COPD and bronchiectasis are characterised by neutrophilic inflammation and share common features and therefore probably some common pathophysiology | Strong association between bronchiectasis and ABPA proves an association with eosinophilic inflammation, therefore association with asthma is highly plausible |
| Coherence | Not fully testable | Not fully testable |
| Experiment | Experimental models of COPD are generally poor and conclusions cannot be drawn | Experimental models of asthma do not show features of bronchiectasis |
| Analogy | COPD fits the model of the vicious cycle of bronchiectasis, being characterised by airway inflammation, infection and airway damage | Asthma fits the model of the vicious cycle of bronchiectasis, being characterised by airway inflammation, infection and airway damage |