iSixSigma

One-Way ANOVA

Six Sigma – iSixSigma Forums Old Forums General One-Way ANOVA

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • #31911

    Cristina
    Participant

    Could we use one-way ANOVA to compare two or more means for samples of data which don’t have equal variances?

    0
    #84589

    John Noguera
    Participant

    Classical ANOVA assumes that the variances are equal. 
    Your choices in Minitab to the following:
    1. Apply a variance stabilizing transformation like ln(Y).
    2. Use a nonparamentric test like Kruskal-Wallis (note here you are testing medians not means).
    3. Limit yourself to 2 samples and apply t-test for unequal variance.
    If you have access to JMP, they have a procedure called Welch’s ANOVA which does not assume equal variance.

    0
    #84603

    Robert Butler
    Participant

      ANOVA can be thought of as a tool that asks the question – “All other things being statistically equal, is there a difference in population means?”  You can use a one way ANOVA to compare more than two means which don’t have equal variances.  The question that you must answer is – how unequal are your population variances?  If they are numerically different but are not statistically different then ANOVA should give you a worthwhile summation of the population mean differences.
      If the variances exhibit enough inequality so that one or more are significantly different you can still use ANOVA and it will probably indicate that there are no significant differences between population means which, in light of the unequal variances, would be the case.  In the latter case your investigation should shift to asking questions concerning the “why” of the significant differences in population variances.
     

    0
    #84643

    John Noguera
    Participant

    Hi Robert,Thanks for your post.  I especially agree with your emphasis on asking why there is a significant difference in variances.  This can lead one to a “golden nugget” X factor; the discovery of such is likely far more valuable than that originally searched for with unequal means.  

    0
Viewing 4 posts - 1 through 4 (of 4 total)

The forum ‘General’ is closed to new topics and replies.