It is a randomized controlled study, meaning that similar patients were assigned at random to either get either Centrum or placebo.

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Because the Framingham Heart Study is not a randomized controlled trial, it's still not clear if vitamin D is the real reason for the link (the vitamin's levels could drop in unhealthy people for other reasons), or if taking a supplement would lower the risk.

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For one, the groups noted the analysis is a retrospective study, not a prospective, randomized controlled clinical trial, which many regard as a gold standard for ferreting out safety concerns.

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All of the studies ran for at least a year, and all were randomized controlled trials, the most rigorous type of study.

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The Cambridge group will begin a randomized controlled trial next month with 104 children involved, similar to the peanut study done already, but with controls, said researcher Dr. Andrew Clark of Cambridge University Hospitals.

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