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Encyclopedia Entry

Alpha, Significance Level of Test

Andrew Noymer

Alpha is a threshold value used to judge whether a test statistic is statistically significant. It is chosen by the researcher. Alpha represents an acceptable probability of a Type I error in a statistical test. Because alpha corresponds to a probability, it can range from 0 to 1. In practice, 0.01, 0.05, and 0.1 are the most commonly used values for alpha, representing a 1%, 5%, and 10% chance of a Type I error occurring (i.e. rejecting the null hypothesis when it is in fact correct). If the p-vahie of a test is equal to or less than the chosen level of alpha, it is deemed statistically significant; otherwise it is not. The typical level of alpha is 0.05, but this is simply a custom and is not based on any statistical science theory or criteria other than conventional practice that has become the accepted standard. Alpha levels of 0.1 ...

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