P Values – CASP

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Language/s: English
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Short Description:

Statistical significance is usually assessed by appeal to a p-value, a probability, which can take any value between 0 and 1 (certain).

Key Concepts addressed:


The p-value is a probability. It can take any value between 0 (impossible) and 1 (certain). It represents the predicted proportion of results which would be more extreme than the one we have observed purely by chance, if there was no true underlying difference between the treatments. An arbitrary threshold for statistical significance must be set, preferably in advance by the trialists, and is usually chosen as p<0.05 (5% or 1 in 20) or sometimes p<0.01 (1% or 1 in 100).