Table 3

Random intercept for SLS COPD placed in distribution of random intercepts of local authorities from CPRD (testing for presence of Hawthorne effect)

VariableCPRD-xGM 2.5th percentileCPRD-xGM median valueCPRD-xGM 97.5th percentileSLS COPD valueSLS COPD percentileUnusual flag
Count of AECOPD episodes*†0.900.991.131.1498.37Yes
Time until first AECOPD episode†‡0.891.001.171.1496.71No
Count of strict definition of AECOPD episodes†§0.361.012.622.5997.36No
Number of days contact with primary care*0.851.001.301.75100Yes
Count of trial-related prescription items*¶0.801.011.261.1787.88No
Treatment switching**††0.660.991.550.694.83No
  • For full definitions of above outcomes, see online supplementary appendix 1.

  • For Poisson models, the random intercepts represent relative rates; for Cox models, they represent HRs; for logistic models, they represent ORs.18 20 SLS COPD is deemed unusual with respect to a particular variable if the random intercept for the SLS lies outside of the 2.5th–97.5th percentile range of the distribution of random intercepts of the local authorities in CPRD. Variables adjusted for, following significant univariate likelihood ratio tests, were sex, age, socioeconomic status, medication group defined from the previous 3 months, history of depression, anxiety, asthma, pneumonia, and gastro-oesophageal and peptic ulcer disease, COPD exacerbation history in the previous 12 months, MRC dyspnoea score, pneumococcal vaccine, FEV1%, FEV1:FVC%, and smoking status.

  • *Poisson model.

  • †AECOPD events were those that met any criteria from the validated algorithm.16

  • ‡Cox model.

  • §The strict definition of AECOPD events was strictly medical codes for acute exacerbation of COPD.

  • ¶Prescription of any treatment counted as usual care in SLS COPD.

  • **Logistic model.

  • ††Switching from one treatment class to another during follow-up.

  • AECOPD, acute exacerbations of COPD; COPD, chronic obstructive pulmonary disease; CPRD-xGM, Clinical Practice Research Datalink outside of Greater Manchester; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; MRC, Medical Research Council; SLS COPD, Salford Lung Study in chronic obstructive pulmonary disease.