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How do I decide where is the threshold to split it into levels? I ran an ANCOVA with a continuous moderator and it is significant, but I am wondering how to transform it to a categorical variable? Is it necessary?

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In general, no it is not necessary and is generally inadvisable as it create loss of information and loss of efficiency. Additionally, it can introduce bias that is remedied by entering the continuous version of the variable into the model as a predictor in addition to the categorical form (meaning the categorical form is not beneficial).

I am unclear of your use of "moderator" in this case. Do you just mean an x-variable or do you mean a variable that interacts with another to change the relationship between Y and another x-variable?

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  • $\begingroup$ Hello, Yes I mean a variable that interacts with X in their effect on Y. A variable I am testing is continuous, and I am not sure if I should - and/or how I can - transform it to categorical (my x is categorical with three groups), and then I have two moderators of which one is continuous and one is categorical with two groups. Is it correct if I just use both as covariates in their original format (continuous and categorical)? $\endgroup$ – Curious Apr 3 at 15:01
  • $\begingroup$ It is correct (as a broadly general rule) to leave continuous as continuous (really interval and ratio scaled measurements as is). $\endgroup$ – LSC Apr 3 at 17:30

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