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Vitamin C supplementation reduces the odds of developing a common cold in Republic of Korea Army recruits: randomised controlled trial
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    Statistical problems in the vitamin C and common cold trial with South Korean army recruits

    The report on vitamin C for preventing the common cold in the Republic of South Korea army recruits by Kim et al. [1] has several statistical problems.

    First, Kim et al. did not follow the intention-to-treat [ITT] approach. Figure 1 shows that 49 participants were excluded because they “stopped intake of vitamin C”, and 84 participants were excluded because they “stopped intake of placebo” [1]. The CONSORT recommendation for ITT analysis states as follows [2, Box 6]: “participants who … did not take all the intended treatment ... exclusion of any participants for such reasons is incompatible with intention-to-treat analysis”.

    Second, Altman et al. pointed out that “The odds ratio should not be interpreted as an approximate relative risk [RR] unless the events are rare in both groups (say, less than 20-30%)”[3]. The common cold is not rare. Over 50% of the participants in the Kim et al. trial had the common cold during the trial period which greatly exceeds the 20-30% limit. Furthermore, there is no need to use the OR as the approximation for RR, because the RR can be calculated from the trial data in Table 1, RR = 0.916 (= 0.538/0.587) [1].

    Third, in their abstract, Kim et al. wrote “the vitamin C group had a 0.80-fold lower risk of getting a common cold” implying that vitamin C decreased the incidence of colds by 20%. However, the correct effect estimate is given by the RR above, which indicates only 8.4% lower risk of colds in the vitamin C group....

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    Conflict of Interest:
    None declared.