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### Can we skip the lower order terms in interactions? [duplicate]

This question is about three-way interaction and the possibility of applying without second lower terms with keeping the main variables in the equation not like the other questions. In fact the other ...
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### Are lower-order interactions a prequisite for three-way interactions in regression analysis? [duplicate]

I'm applying growth curve modelling and am interested in modelling a three-way interaction between predictors A x B x C. From what I remember a condition for modelling three-way interactions in ...
88k views

### Including the interaction but not the main effects in a model

Is it ever valid to include a two-way interaction in a model without including the main effects? What if your hypothesis is only about the interaction, do you still need to include the main effects?
63k views

### Does it make sense to add a quadratic term but not the linear term to a model?

I have a (mixed) model in which one of my predictors should a priori only be quadratically related to the predictor (due to the experimental manipulation). Hence, I would like to add only the ...
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### Interactions terms and higher order polynomials

If I were interested in fitting two-way interactions between a linear explanatory variable $a$ and another explanatory variable $b$ that has a quadratic relationship with the dependent variable $y$, ...
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### Does triple interaction need to include all main effect variables?

I have a triple interaction: AxBxD, where A and B are continuous variables and D is a dummy. My regression is Y = A + B + AxB + AxD + AxBxD In this case, do I HAVE TO include BxD also? In theory here ...
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### Testing for moderation with continuous vs. categorical moderators

I am testing an interaction effect where $X$ and $Y$ are continuous variable and $M$ (Moderator) is a categorical variable (effects coding $+1$, $-1$). I have no clue about how to do a post-hoc ...
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### Whether to include $x$ and $x^2$ in regression model examining diminishing returns when only $x^2$ is significant?

I have a data set with sales as $Y$ and total retail shelf space ($x$). I want to investigate the diminishing return, i.e. whether additional shelf space would contribute to more sales. I did ...
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### Interaction Terms and Logit Models

I have read the previous discussions on interaction effects and main effects, and I have a question on the subject. I am running a destination choice model (multinomial logit), and I have one ...
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### What is the meaning of the beta for the interaction between continuous variables in a linear mixed-model?

If I create a mixed-effects linear regression model similar to the following (using the lme4 package in R), where all of the fixed effect variables are continuous: ...
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### Testing significant differences between regressions in R

I am running several phylogenetic least squares analyses in R, where I'm taking an existing data set for several species, and adding two new species for which I have data. I want to do is test whether ...
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### Why lasso yield a higher mse then ridge?

I do a rige and lasso regression on a train data set and get the lambdas via cross validation and evalute the prediction ...
398 views

### To exclude a main effect that is not of interest in ANCOVA, or not? [duplicate]

Possible Duplicate: Including the interaction but not the main effects in a model I have an experimental design with pretest and posttest. Two groups, experimental and control group. I am ...