The answer is explicitly stated in the passage: "In the British Medical Journal the other day, there was a report of a meeting. ‘The Common Cold - Fact and Fancy’, at which one of the speakers reported a study of colds made in Cirencester over the last five years. Three hundred and fifty volunteers had kept diary records of their colds..." So, the volunteers in Cirencester kept a record of their colds through personal diaries, as mentioned in the passage.