Non-response in telephone surveys of COPD patients does not introduce bias

We examined the telephone response rates in a 2-year study of patients with COPD to identify factors associated with non-response. A total of 381 patients received monthly telephone calls to assess symptoms and treatment in 2007 and 2008. A total of 9019 calls were made over 24 months, of which 73% were answered. The highest response rate was obtained in February 2007 (81%), and the lowest in July 2008 (48%). The monthly response rate was lower in the second year of follow up. There were 39 patients (10%) who were less frequent responders, with 10 or fewer responses. Less frequent responders were more likely to be current smokers and have hypoxaemia. The median number of answered calls was 18 (interquartile range 16-20). In bivariate models the median number of responses was significantly lower in subjects with chronic cough. In multivariate models neither demographic nor disease characteristics were significantly associated with non-response. We found less frequent responders to monthly telephone calls to be similar in characteristics to frequent responders. This suggests that non-response does not necessarily introduce bias in telephone surveys of patients with COPD.