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What does “statistically significant” mean?

‘To be honest, it’s a tricky idea. It can tell us if the difference between a drug and a placebo or between the life expectancies of two groups of people, for example, could be just down to chance . . . It means that a difference as large as the one observed is unlikely to have occurred by chance alone.

Statisticians use standard levels of “unlikely”. Commonly they use significant at the 5% level (sometimes written as p=0.05). In this case a difference is said to be ‘significant’ because it has a less than 1 in 20 probability of occurring if all that is going on is chance.’

Spiegelhalter D, quoted in: Making Sense of Statistics. 2010. www.senseaboutscience.org

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