Hi Kathryn,
They are both right, because they are saying the same thing, but in a
different way.
First, you could look at each group individually. You would look for
outliers, which means you would be looking for individuals who are far
from the mean.
Second, you could look at residuals. The residual is calculated as
(actual score) - (predicted score).
In a one way anova, your predicted score for each group is the group
mean. So you would be looking for high residuals, which means people
are far from the mean.
Looking at groups no longer works as soon as you have a continuous
predictor in there, and do an ANCOVA, or a regression.
ANOVA, ANCOVA and regression are all the same thing really, so there's
always equivalence between them. (I tend never to use ANOVA/ANCOVA, I
do everything with regression, because then I'm always using the same
approach, and always doing the same thing.)
The slight advantage of ANOVA is that it's easier to deal with
non-homogeneity of variance, because you can do the Brown or Welch (or
Satterthwaite) correction . However there's a lot of irrelevant
nonsense talked about that assumption, because if you have equal
sample sizes, it's irrelevant. You can correct by using what's
sometimes called robust estimation, or sometimes called using a
sandwich estimator, but it's fiddly in SPSS.
Jeremy
On 28/06/07, Kathryn Jane Gardner <[log in to unmask]> wrote:
> Dear list,
>
> I am hoping that someone can clarify the following for me regarding one-way ANOVAs. In all stats books I have read, the reader is advised to pre-screen the DV for outliers and normality by using boxplots and histograms separately for each group of the IV. However, I have read/heard elsewhere that the assumption is normality of the residuals (as in regression). If this is the case then it would appear the all of these stats books are not providing the correct advice and it is pointless pre-screening the data.
>
> If the assumption is normality of the residuals, I assume that this applies in one-way ANOVA also? However, there is no option to save residuals in SPSS's one-way ANOVA (I'm not sure why). The alternative would be to run the one-way ANOVA via the Multivariate ANOVA option and save the residuals, then plot histograms, boxplots and normality plots etc based on these residuals.
>
> Thanks
> Kathryn
>
--
Jeremy Miles
Learning statistics blog: www.jeremymiles.co.uk/learningstats
Psychology Research Methods Wiki: www.researchmethodsinpsychology.com
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