text: Although neither the CAT nor the SGRQ are dyspnea- 
  speciﬁc tools, both are recommended by the Global Initia- 
  tive for Chronic Obstructive Lung Disease to evaluate 
  symptoms, including dyspnea,20 and both yield a richer 
  assessment of dyspnea than the modiﬁed Medical 
  Research Council breathlessness scale.20 Fifteen questions 
  were taken from the CAT and SGRQ questionnaires that 
  referred to individuals’ experiences with dyspnea, and a 
  composite measure of dyspnea impact using a weighted 
  sum of the responses to the 15 questions was constructed. 
  Questions were coded so that larger values indicate more 
  impactful dyspnea. Weights used for question responses 
  in calculating the dyspnea impact assessment measure 
  were those of the ﬁrst component of a principal compo- 
  nent analysis (PCA) based on the covariance matrix of 
  question responses. Questions with multiple responses 
  and ordinal structure are individually more informative 
  and thus were accorded higher weight than individual 
  true-false questions. No additional PCA component was 
  anticipated a priori to be material for our investigation, 
  and an eigenvalue analysis of the PCA was conducted to 
  verify this assumption. 