Table 3.

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

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

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.