Dear R-lang,
I have a question regarding the interpretation of different coding
schemes in a regression model. My experiment compares 8 different levels
of a single independent variable. Additionally, it is possible to
conceive of 4 of these 8 levels as a mini-2x2 within the 8 level variable:
Cells: A, B, C, D, E-1, E-2, F-1, F-2
I hypothesize the following ordering (which is substantiated by the
overall means in the different conditions):
ABC>D>E1>F1>E2>F2
Additionally, I hypothesize that E>F and 1>2. I have implemented 2
different models to test these hypotheses and I would like to hear your
take on what the correct interpretation of the results is. In model 1
have have Helmert-coded the hypothesized ordering in terms of the
following five contrasts:
ABCvDE1F1E2F2
DvE1F1E2F2
E1vF1E2F2
F1vE2F2
E2vF2
All Helmert-contrasts are significant. In model 2 I have used the
following contrasts instead:
ABCvDE1F1E2F2
DvE1F1E2F2
EvF
1v2
All contrasts except EvF are significant. Collinearity is minimal in
both models (all correlations below |.2|)
Is it fair to say that the ABC>D>E1>F1>E2>F2 has been substantiated by
the experiment and thus infer that E>F, even though the EvF contrast is
not significant in a differently coded model?
Thank you very much in advance for your help,
Best,
Peter Graff
