what's anova? 方差分析

In statistics, analysis of variance (ANOVA) is a collection of statistical models, and their associated procedures, in which the observed variance in a particular variable is partitioned into components attributable to different sources of variation. In its simplest form ANOVA provides a statistical test of whether or not the means of several groups are all equal, and therefore generalizes t-test to more than two groups. Doing multiple two-sample t-tests would result in an increased chance of committing atype I error. For this reason, ANOVAs are useful in comparing two, three or more means.

http://en.wikipedia.org/wiki/Analysis_of_variance

http://wenku.baidu.com/view/ee23267b31b765ce05081430.html

http://baike.baidu.com/view/786804.htm