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I recently conducted a study with 4 within-subject conditions, divided over two sessions. My independent variable is degree of emotional eating (as measured by food intake after a negative mood manipulation), my dependent variables are food intake after some other conditions (positive mood, exposure to food, and a control condition). I want to investigate whether high emotional eaters overeat after a variety of cues (like positive mood and exposure). I hypothesize that emotional eaters (i.e., those who eat most after the negative mood manipulation) will consume more food than low-emotional eaters after each experimental condition (positive mood and exposure), but not control. In addition, I expect the high emotional eaters to eat more after positive mood and exposure compared to control, while I don't expect any differences between conditions in the low-emotional eaters.

I know how to analyze my data using a median-split to categorize high and low emotional eaters and follow this by a mixed ANOVA. However, I would like to avoid median-split and treat my independent variable as a continuous variable. But, I'm stuck on how to proceed. In previous studies in which I wanted to treat emotional eating as a continuous variable, I performed a moderated regression analysis (+1 and -1 SD). However, those studies had between subject designs, and I'm confused on how to do these analyses with my current design, if that's even possible. Alternatively, I received some suggestions for doing linear mixed model analyses.

Could someone point me in the right direction? Thanks so much for your help!

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If you use R and the nlme package, the syntax would be:

library(nlme)
m <- lme(dv ~ iv*emo_eating, random=~1|id, data=d)
summary(m)

where emo_eating is the continuous predictor, id is the subject-ID variable, and d is your data stored as a data frame. I am assuming that iv is a fixed effect. The summary(m) will give you regression output.

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