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Table 3 Sample size calculations for estimating the overall frequency of IID via self-report telephone survey.

From: Methods for determining disease burden and calibrating national surveillance data in the United Kingdom: the second study of infectious intestinal disease in the community (IID2 study)

Duration of recall period Incidence in original IID recall Questionnaire Widest acceptable CI limit Number needed to survey
28 days 6% 4% 500
7 days 1.5% 1% 2500
  1. Note: Based on an expected frequency of IID of 6%, with a 95% Confidence Interval (CI) of 4% to 8%. Allowing for differentials in response rate the number needed to survey was increased by 20% i.e. to 600 for recall over one month and 3000 for recall over one week.