ANOVA à un facteur : tailles d'échantillons inégales

Sample sizes directly affect both variance estimates used to calculate the F statistic. When sample sizes are unequal, both the variance between samples and the variance within samples are weighted by sample size, meaning larger samples have greater influence on these estimates. This weighting ensures accurate representation of each group's contribution to the overall analysis.

The F statistic is computed as the ratio of variance between samples to variance within samples, with both estimates weighted by sample size. The formula incorporates the combined mean of all observations, individual sample means, and sample sizes for each group. This weighted approach accounts for the different number of observations in each sample when comparing group means.

The null hypothesis states that all group means are equal, while the alternative hypothesis states that at least one group mean differs from the others. The P-value from the F statistic determines whether to reject the null hypothesis. If rejected, it indicates significant differences exist among the groups, though it does not specify which groups differ.

After rejecting the null hypothesis, you can use multiple comparison tests to identify specific differences between groups. Alternative approaches include constructing box plots for visual comparison or building confidence intervals around sample means. These follow-up methods pinpoint which particular groups have significantly different means from one another.

Unequal sample sizes create weighted variance estimates because the formulas incorporate sample size when calculating both between-group and within-group variance. Larger samples contribute more weight to these estimates, affecting the final F statistic value. This weighting mechanism ensures that groups with more observations have proportionally greater influence on the statistical test.

Calculations become complicated because the variance estimates must be weighted by sample size, requiring more complex formulas than the equal sample case. The different group sizes mean each observation does not contribute equally to the overall analysis. This weighting adds computational steps but ensures statistically valid comparisons across groups of different sizes.