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Table 1 Evolution of the study groups in the follow-up study (adapted from reference 12)

From: A controlled survey of less typical long-term consequences after an extensive waterborne epidemic

  Contaminated group Control group
Population 2007 9 538 27 259
Original sample size 1 021a 1 000
Responded to the first study N (%) 808 (79%) 598 (60%)
Sample size in the follow-up study (i.e., those who gave permission to be contacted again) 615 343
Responded to the follow-up study N (%) % of the original sample 323 (53%) 32% 186 (54%) 19%
Having had epidemic-related gastroenteritis during the epidemic, according to the follow-up study 174 (54%) 6 (3%)
  1. aThe original sample size was 1000 persons in all groups. Assessment of the contaminated area became more precise later, and 21 participants were shifted to the contaminated group