The P value is a probability, with a value ranging from zero to one, that answers this question:
In an experiment of this size, if the populations really have the same mean, what is the probability of observing at least as large a difference between sample means as was, in fact, observed?
aus: GraphPad GraphPad Prism 5.0 Statistics Guide
UPDATE:
hahaa, there is more:
Compare, for example, the statement "The observed differences could occur 5% of the time if the true effect is zero" with the statement "The probability that the true effect is zero is 5%". Not only is the latter statement wrong, it does not match the scientific question, which should be to estimate, at a given probability, the minimum size of the effect. Another common variation is to report "no differences between groups" on the basis of t-tests that check for a difference only between the group means. Nature
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