Introduction | Sample size | Probability | Credits |

When we perform a chi-square test, the null hypothesis is that the observed value minus the expected value equals zero, which is the same thing as saying that the observed value equals the expected value. Of course, they are not often the same, and the test allows us to determine the probability that the difference arose by chance alone. It is common in biology to choose a probability of 5% as a cut-off. If the probability that the differences arose by chance alone is greater than 5% (p > .05), we accept that the observed and expected are the same. If the probability is less than 5% (p < .05), we decide that some factor other than chance has caused the difference, i.e., that it is real. Nevertheless, the same rules of probability tell us that if we choose the 5% "confidence level", we will |

jcclark@csupomona.edu

© 2001 by Curtis Clark

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© 2001 by Curtis Clark