The likelihood that an event will occur. When looking at differences between data samples, statistical techniques are used to determine if the differences are likely to reflect real differences in the whole group from which the sample is drawn, or if they are simply the result of random variation in the samples. It is the probability that a test statistic would be as extreme as, or more extreme than that observed if the null hypothesis were true. The letter P, followed by the abbreviation n.s. (not significant) or by the symbol < (less than) and a decimal notation such as 0.01, 0.05, is a statement of the probability that the difference observed could have occurred by chance, if the groups are really alike, i.e. under the null hypothesis. Although investigators may choose their own significance levels in most studies, a result whose probability value is less than 5% (P<0.05) or 1% (P<0.01) is considered sufficiently unlikely to have occurred by chance to justify the designation “statistically significant”. For example, a probability (or P value) of 1% indicates that the differences observed would have occurred by chance in one out of a hundred samples drawn from the same data.
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