We read with interest the letter from Dr. López-Campos and Dr. Alcázar1 commenting on the EPI-SCAN II2 study, and we thank them for the opportunity to discuss three relevant aspects of population research in general and COPD in particular.

Firstly, we fully agree that it is important to encourage the debate between airflow limitation and clinical COPD. In this respect, EPI-SCAN II used the same COPD criteria universally accepted in epidemiological studies, and that were also used in EPI-SCAN I in order to compare results. Comparisons between participants diagnosed with COPD according to these criteria or by clinical criteria, in addition to spirometry, is one of the secondary objectives of the study that will be analyzed in a future publication.

Secondly, the entire methodology of the EPI-SCAN II study, including its sampling strategy, is summarized in the www.clinicaltrials.gov repository, under number NCT03028207, and in the methods section of our article. It is, furthermore, described in more detail in the protocol previously published in Archivos de Bronconeumología.3

Finally, with regard to possible selection biases, we admit that the methodology we used may not be completely bias-free, but it was the best alternative available to conduct a study of these dimensions. Performing spirometries in the general population is not easy. Choosing major cities or provincial capitals does not in itself constitute a selection bias, but was instead a reasoned methodological decision, since the practical aspects of inviting people who live at some distance from a hospital to participate altruistically in a research study are much more complex and expensive than conducting the study in the inhabitants of an urban capital, where research teams and certified lung function laboratories are concentrated. We agree that telephone sampling may potentially lead to a selection bias, but given the current legal impossibility of performing any kind of census sampling we felt postal or internet sampling alternatives were no better. These limitations have already been recognized in the text of our article: “…The majority of areas studied were urban, and although more than 90% of the Spanish population lives in cities, the population burden of disease in rural areas is expected to be high but underrepresented here”. There is little doubt that the problem of COPD and all other chronic diseases associated with smoking and aging in the so-called “empty Spain” is a specific issue that requires a different approach that was not among the objectives of EPI-SCAN II.