Fig. 1

Illustration of the total effect and direct effect of bronchiectasis on COPD. (A) The total effect of bronchiectasis on COPD is equal to β(XY). (B) The direct effect of bronchiectasis on COPD is the effect bronchiectasis has on COPD not via any other exposure variables, which is equal to β (XY1). The total effect bronchiectasis has on COPD is the effect of bronchiectasis on COPD directly plus the effect of bronchiectasis on COPD via eosinophil or neutrophil cell or biological sex (age adjusted) or COPD/asthma/ILD related infections or smoking status, which is equal to β(XY1) + β (XZ)*β(ZY). COPD, Chronic Obstructive Pulmonary Disease. ILD, Interstitial Lung Disease