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Table 1 Distribution of the design variables

From: Diverging trends of chronic bronchitis and smoking habits between 1998 and 2010

  ISAYA (1998/2000) N = 25,969 GEIRD (2007/2010) N = 18,357 p-value*
N° of responders (response rate, %) 18,873 (72.7) 10,494 (57.2) <0.001
Season at time of the interview, %    <0.001
 spring 18.5 41.0  
 summer 16.4 22.1  
 autumn 40.4 25.7  
 winter 24.7 11.2  
Phone contact, % 27.9 11.8 <0.001
Females, % 50.7 52.3 0.008
Age (years), mean (sd) 33.1 (6.9) 34.7 (7.1) <0.001
  1. * p-value obtained by Pearson’s chi-squared test and two-sample t test with unequal variances.