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Volume 8, Number 1—January 2002

Research

Participant Blinding and Gastrointestinal Illness in a Randomized, Controlled Trial of an In-Home Drinking Water Intervention

John M. Colford*†Comments to Author , Judy R. Rees*†, Timothy J. Wade*†, Asheena Khalakdina*†, Joan F. Hilton‡, Isaac J. Ergas*, Susan Burns*, Anne Benker*, Catherine Ma, Cliff Bowen, Daniel C. Mills, Duc J. Vugia, Dennis D. Juranek¶, and Deborah A. Levy¶
Author affiliations: *University of California Berkeley, Berkeley, California, USA; †California Emerging Infections Program, Berkeley, California, USA; ‡University of California San Francisco, School of Medicine, San Francisco, California, USA §California Department of Health Services, Berkeley, California, USA; ¶Centers for Disease Control and Prevention, Atlanta, Georgia, USA;

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Table 4

Rates of highly credible gastrointestinal illness (HCGI) episodes and incidence rate ratios (IRR)a for all respondents and index respondents, by respondent guess about device assignment, week 16,b Pilot Water Evaluation Trial

Guess about device assignment, 16 weeks All respondents Index respondents
Sham device Active device Sham device Active device
Guess = “Sham”
Episodes of HCGI 4 4 3 1
Person-time (person-years) 3 3 1 1
No. of respondents 12 12 5 5
Rate (95% CI) 1.2 (0.3–4.8) 1.1 (0.5–2.7) 2.1 (0.7–6.6) 0.7 (0.1–4.8)
IRR for sham vs. active (95% CI) 1.0 (0.2–5.4) 3.16 (0.3–30.4)
Guess = “Active”
Episodes of HCGI 30 30 6 15
Person-time (person-years) 8 12 4 5
No. of respondents 30 43 13 19
Rate (95% CI) 3.6 (2.1–6.3) 2.5 (1.3–4.5) 1.6 (0.7–3.6) 2.8 (1.7–4.6)
IRR for sham vs. active (95% CI) 1.5 (0.7–3.3) 0.6 (0.2–1.5)
Guess = “Don’t know”
Episodes of HCGI 18 15 10 6
Person-time (person-years) 8 6 4 3
No. of respondents 27 21 13 9
Rate (95% CI) 2.4 (1.2–4.6) 2.5 (1.1–5.5) 2.7 (1.5–5.1) 2.3 (1.0–5.2)
IRR for sham vs.active (95% CI) 1.0 (0.3–2.7) 1.2 (0.4–3.3)
All guesses
Episodes of HCGI 52 49 19 22
Person-time (person-years) 19 22 9 9
No. of respondents 69 76 31 33
Rate (95% CI) 2.7 (1.7–4.3) 2.2 (1.5–3.4) 2.2 (1.4–3.4) 2.3 (1.5–3.5)
IRR for sham vs.active (95% CI) 1.2 (0.6–2.2) 0.9 (0.5–1.7)
No guess given
Episodes of HCGI 12 5 5 2
Person-time (person-years) 4 2 1 1
No. of respondents 21 13 8 5
Rate (95% CI) 3.3 (1.1–9.7) 2.7 (1.1–6.9) 5.4 (2.6–11.3) 3.0 (0.7–11.8)
IRR for sham vs. active (95% CI) 1.2 (0.3–5.1) 1.8 (0.4–8.8)

aRates of HCGI and IRR were calculated by Poisson regression and were adjusted for the intrahousehold correlation introduced by the sampling design.

bRespondents for the blinding questionnaires were all aged >12 years.

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