Re: follow-up question to: ANOVA and HOV

Posted by Marta García-Granero on Jun 28, 2006; 4:15pm
URL: http://spssx-discussion.165.s1.nabble.com/SPSS-MVA-tp1069196p1069202.html

Hi Deborah,

DP> My designs are not Oneway ANOVAs but balanced two-way ANOVAs with
DP> replication (e.g. 3 sites, 4 depths and 4 replicates).   P was generally
DP> less than 0.00001 for the interaction, the site effect and the depth
DP> effect.

The presence of significant interaction modifates the scenario quite a
lot: you can't test main effects & post-hoc for main effects if
interaction is present. You must continue your analysis with a series
of partial tests (you split your dataset by one factor and analyze the
other factor with ONEWAY ANOVA, then you split for the second and
analyze the first). If you need extra information on how to proceed
tell me (but on the list, please).

DP> For other experiments no transformations gave equality of variances.  In
DP> these cases if the sites are looked at separately i.e. just looking at the
DP> data for the change of depth within each site then I do get equality of
DP> variances.  For these experiments is it better to just look at each site
DP> separately using a Oneway ANOVA? (The sites are actually so different for
DP> the variable I am looking at).

For balanced designs, the efect of the lack of HOV is minimal. Perform
an extra check comparing the smallest SD  with the biggest SD, if the
ratio biggest/smallest is below 2, then the lack of HOV (whatever
Levene test says) is not really important.

DP> When is Tukey better to use than the Tamhane test?  I mean what if P=0.06
DP> for Leven HOV test?

If you suspect lack of HOV (if Levene test is non significant, 0.06
f.i., but ratio of SD -bigger/smaller- is over 2), then Tamhane test
will control it better. But remember that you can't use post-hoc tests
for main effects when interaction is present.

DP> For ANOVA does one need to check that the data as well as the residuals
DP> follow a normal distribution of is just the residuals sufficient?

Only the residuals.

Regards,
Marta