We thank the authors for their interest in our research1 which has attracted both professional and public media interest.2–4 Certainly, we accept that epidemiological studies can be at risk of ‘ecological fallacy’ and cannot adjust for all possible confounders. We make no claim that association proves causality. However, in offering a generic note of caution, Ford et al provide no specific hypothesis or insight to explain why the associations illustrated in our particular study could be explained by unacknowledged bias or an artefact of data aggregation. Taking the logic from their example, the counter hypothesis for our findings would be that people who are destined for a poor cancer outcome were selectively choosing to register at a general practice with a low gastroscopy rate—this seems unlikely.

We took great care to explore potential bias in our approach to data aggregation by undertaking extensive sensitivity analyses.5 Each case of oesophagogastric cancer was assigned a range of alternative …