# Questions tagged [interaction]

A situation where the effect of an explanatory variable may depend on the value of another explanatory variable.

185 questions
17answers
66k 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?
9answers
48k views

### Obtaining knowledge from a random forest

Random forests are considered to be black boxes, but recently I was thinking what knowledge can be obtained from a random forest? The most obvious thing is the importance of the variables, in the ...
7answers
37k views

### Do all interactions terms need their individual terms in regression model?

I am actually reviewing a manuscript where the authors compare 5-6 logit regression models with AIC. However, some of the models have interaction terms without including the individual covariate terms....
4answers
24k views

### What if interaction wipes out my direct effects in regression?

In a regression, the interaction term wipes out both related direct effects. Do I drop the interaction or report the outcome? The interaction was not part of the original hypothesis.
1answer
35k views

### What is the non-parametric equivalent of a two-way ANOVA that can include interactions?

Hi I am trying to find the non-parametric equivalent of a two-way ANOVA (3x4 design) which is capable of including interactions. From my reading in Zar 1984 "Biostatistical analysis" this is possible ...
1answer
5k views

### Interpretation of interaction term

I have a model: $$\ln({\rm earnings}) = a+b_1{\rm female}+b_2{\rm white}+b_3{\rm female}\times{\rm white}$$ ${\rm female}$ and ${\rm white}$ are dummy variables. I have interpreted $b_1$ and $b_2$: ...
1answer
8k views

### 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 ...
3answers
40k views

### Interpreting interaction terms in logit regression with categorical variables

I have data from a survey experiment in which respondents were randomly assigned to one of four groups: ...
1answer
8k views

### Interaction in generalized linear model

I have 2 continuous variables as my predictors and the interaction between them, so 3 effects all together (when I center my predictors only the interaction is significant). I am using a binary logit ...
1answer
9k views

### Interpreting the regression output from a mixed model when interactions between categorical variables are included

I have a question about my use of a mixed model/lmer. The basic model is this: lmer(DV ~ group * condition + (1|pptid), data= df) Group and condition are both ...
2answers
7k views

### In three-way ANOVA, how to interpret the three-way interaction?

I have a significant three-way interaction (with no significant two-way interactions and no significant main effects). To begin with I have followed this up with two two-way ANOVAs (one at each level ...
1answer
163 views

### Can I analyze or model a conditional correlation?

In my research I'm looking at the correlation between self-harm and aggression (both continuous). Now, I also have some variables (e.g. depressive symptoms; also continuous) which I do believe ...
3answers
35k views

### Why could centering independent variables change the main effects with moderation?

I have a question related to multiple regression and interaction, inspired by this CV thread: Interaction term using centered variables hierarchical regression analysis? What variables should we ...
5answers
28k views

### What are best practices in identifying interaction effects?

Other than literally testing each possible combination of variable(s) in a model (x1:x2 or x1*x2 ... xn-1 * xn). How do you ...
4answers
19k views

### “Moderation” versus “interaction”?

I have come across these two terms which are used interchangeably in many contexts. Basically, a moderator (M) is a factor that impacts on the relationship between X and Y. Moderation analysis is ...
3answers
18k views

### How to investigate a 3-way interaction?

There is a significant 3-way interaction in a data-set I'm working with. The interaction involves both categorical and quantitative variables. I have been pointed towards simple slopes and this ...
2answers
4k views

### ANCOVA and its disturbing assumptions

It is possible to test for interaction in Analysis of Covariance (ANCOVA) between the independent variable and the co-variate, but isn't “the homogeneity of regression slopes” an assumption that ...
2answers
6k views

### How to test whether a regression coefficient is moderated by a grouping variable?

I have a regression done on two groups of the sample based on a moderating variable (say gender). I'm doing a simple test for the moderating effect by checking whether the significance of the ...
1answer
27k views

### The main effect will be non-significant if the interaction is significant? [duplicate]

I am using linear mixed models to identify important factors, and it turns out that: A: significant B: not significant ...
1answer
16k views

### Is interaction possible between two continuous variables?

All of my variables are continuous. There are no levels. Is it possible to even have interaction between the variables?
5answers
21k views

### What is a block in experimental design?

I have two questions about the notion of block in experimental design : (1) What is the difference between a block and a factor ? (2) I tried to read some books but something is not clear: it seems ...
1answer
607 views

### Investigating interaction

Please I need to check for interaction before building an explanatory model (logistic regression). I have 16 interaction terms in total. Please how what is the best way to go about it. Will I need to ...
1answer
3k views

### Interpreta​tion of main effect when interactio​n term is significan​t (ex. lme)

As an example I use Pinheiro, J. C. & Bates, D. M. 2000. Mixed-effects models in S and S-PLUS. Springer, New York. page 225. Rats whose body mass has been measured are fed by 3 different diets ...
2answers
5k views

### Interpreting main effect and interaction

I am doing a simple marketing project that has the following types of variables: X1 - continuous (e.g. income) X2 - categorical (e.g. gender) Y - continuous (e.g. number of a product type purchased ...
1answer
3k views

### How to interpret interaction continuous variables in logistic regression?

I am struggling to understand and interpret the interaction term in a logistic regression. The explanatory variables are temperature (categorical), ...
1answer
132 views

### How to code values for males when they're only meaningful for females? [duplicate]

I'm building a predictive model for a medical condition that happens in both men and women. Physicians have reported that in some cases for women, their menstrual history seems to be a risk factor, ...
3answers
47k views

### How to interpret main effects when the interaction effect is not significant?

I ran a Generalized Linear Mixed Model in R and included an interaction effect between two predictors. The interaction was not significant, but the main effects (the two predictors) both were. Now ...
3answers
15k views

### How does the interpretation of main effects in a Two-Way ANOVA change depending on whether the interaction effect is significant?

As far as I can tell this question has never been asked before. There are several questions that touch on related issues, but as far as I can see none of them have provided a definitive answer to this ...
2answers
16k views

### How can one plot continuous by continuous interactions in ggplot2?

Let's say I have data: ...
1answer
15k views

### Interpreting coefficients of an interaction between categorical and continuous variable

I have a question about the interpretation of the coefficients of an interaction between continuous and categorical variable. here is my model: ...
1answer
7k views

### Interaction term using centered variables hierarchical regression analysis? What variables should we center?

I'm running a hierarchical regression analysis and I have some little doubts: Do we calculate the interaction term using the centered variables? Do we have to center ALL the continuous variables we ...
1answer
4k views

### How do I create and interpret an interaction plot in ggplot2?

I am trying to make an interaction plot for this set of data, but I want to make it with ggplot2. I am trying to predict VP using the predictors G and P, (these are columns in the dataset which I have ...
1answer
2k views

### Is it meaningful to identify interaction effect when a main effect in a model (main effects only, without interaction terms) is not significant?

Let's say there's one DV (Y) and three IVs (X1, X2, X3), ...
2answers
14k views

### How do you plot an interaction between a factor and a continous covariate?

I would like to plot on the same graph the interaction between my continuous predictor and my categorical moderator. I know how to do it when both are categorical (factor interaction), but don't ...
1answer
4k views

### How to interpret the significant interaction of two non-significant main predictors?

I have two non-significant main binary variables in my binary logistic regression model, but their interaction is significant. The variables are centered and no multicollinearity is the case (all VIFs ...
1answer
590 views

### ols vs logistic regression, does the interpretation of interaction differs?

My question concerns the interpretation of interaction in ols vs logistic regression. Let us imagine that I have two independent variables, $y_{1}$ is the probability of watching TV at least once a ...
1answer
529 views

### Interpretation of interactions between categorical data

I have data that looks a bit like this ...
1answer
5k views

### Interpreting a significant 2-way interaction with post-hoc tests

I'm analyzing reaction time data from a repeated measures ANOVA with the following design: Factor 1 (between-group): GROUP (controls, clinical) Factor 2 (within-...
1answer
3k views

### Cohen's d for 2x2 interaction

I have a significant 2x2 interaction in a between-subjects design. I would normally report omega-squared as my effect-size measure, but I've been asked to provide Cohen's d instead? Can Cohen's d be ...
2answers
6k views

### 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 ...
1answer
115 views

### Prediction using fixed effects in glmm

I have the following generalized linear mixed effects model (mcmcglmm in R) with data based on this paper. Sex is a two level factor (M or F), ...
2answers
142k views

### Different ways to write interaction terms in lm?

I have a question about which is the best way to specify an interaction in a regression model. Consider the following data: ...
2answers
2k views

### Interpretation of multiple logistic regression with interactions in R

I am trying to look at whether 2 variables (one dichotomous categorical and one continuous) predict the occurrence of a dichotomous categorical dependent variable. ...
2answers
9k views

### Two negative main effects yet positive interaction effect?

I have two main effects, V1 and V2. The effects of V1 and V2 on the response variables are negative. However, for some reason I am getting positive coefficient for the interaction term V1*V2. How can ...
2answers
3k views

### Interaction suppresses the main effect? How to interpret it?

I have a simple model without interaction and it stated significant effect for all the explanatory variables (continuous variable rok and categorical variables obdobi (levels hn and nehn) and kraj: <...
1answer
8k views

### How to calculate the standard error of the marginal effects in interactions (robust regression)?

what I am interested in learning is how to calculate the std error of the marginal effects of a X variable when it is part of an interaction, especially in robust regression. There are tipically two ...
1answer
10k views

### Cox proportional hazard model and interpretation of coefficients when higher case interaction is involved

Here is the summary-output of the Coxph-model I used (I used R and the output is based on the best final model i.e. all significant explanatory variables and their interactions are included): ...
3answers
7k views

### Interaction term in linear regression

I have two variables, and the relationship between them looks like that in this plot: ggplot(mtcars, aes(wt, mpg)) + geom_point() The relationship between ...
2answers
10k views

### Calculating and interpreting effect sizes for interaction terms

Can an effect size be calculated for an interaction effect in general and more specifically using the F-statistic and its associated degrees of freedom? If yes, should this be done and what is the ...
4answers
8k views

### Can the interaction term of two insignificant coefficients be significant?

Lets say I have a linear regression with two numeric explanatory variables: A and B. Consider the following scenarios: A and B are both insignificant A is significant, B is insignificant; or the ...