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ANOVA Query

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  • #49464

    Swaggerty
    Participant

    Doing an ANOVA to compare equality of means for more than 2 samples will tell me if their means are equal or not. However,taking 3 samples, if 2 sample means are equal, but the third is not…is there any way to determine which is the sample with the unequal mean?

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    #169076

    Robert Butler
    Participant

    Yes, it’s called pairwise comparisons of means.  Somewhere in your computer code for the ANOVA there should be a request for this.  In the code I use it is buried under a request for least squares means and the request is pdiff -short for pairwise difference comparison. Under that same heading should be a comment to the effect that you will have to choose some kind of adjustment to the alpha level because you are now doing multiple tests on the same data.  Choices should be such things as Bonferroni, Tukey-Kramer, Dunnett, etc.

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    #169189

    Fontanilla
    Participant

    Using Minitab, it’s too easy to get pairwise comparison.  It’s just a check-box.
    Now, if you don’t have Minitab or other software, you can still compare the means both analytically and graphically.  As always, I recommend you start with the graphical comparison.  It should be quite obvious which mean is different when graphed.  If it’s not, and you need to know which is different, there are (2) points I’d like to make here: 
    First, if the difference is so small that you cannot detect it graphically, go back and re-evaluate what you’ve really learned (think practically).  Most of the time, we are looking for big shifts or differences in processes as a result of a bad factor, or as a result of a corrective action applied.  If your result is so small, you may not have the right factors involved or they may not be at the right levels.
    Second, get yourself Golnick’s Cartoon Guide to Statistics.  It’s really simple but absolutely the most effective book I’ve seen to nail these concepts down.  It provides you the basis for the calculations and the formulae to do the analysis.

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