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In linear models and particularly in ANOVA, a contrast is a linear combination of parameters with coefficients summing up to zero. It is used to test the corresponding null hypothesis. Contrasts are especially often used with categorical predictors (factors) to make comparisons among the groups (categories). [See also tag 'categorical-encoding']

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Planned comparisons are usually powerful but you´ll need to check if Helmert contrast are a type of ORTHOGONAL contrast (independent). If Helmert contrast is in fact a non-orthogonal contrast then you …
answered Aug 24 '12 by Damian
0
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Yes you need to adjust alpha, especially if you do planned comparisons that are non-orthogonal. If you have set the contrasts in an ANOVA and tested the orthogonality between them, you can descompose …
answered Jun 8 '12 by Damian