When to use "With" & When to Use"By"

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I am using a random intercept model, and I have a question regarding SPSS
syntax. I would like see how a covariate is related to the dependent
variable, however, also control for the covariate effects on the
participants in the model.

To do so, in SPSS syntax, can I enter covariates following the “BY”
function and also add covariates in the FIXED function? By using this
formula, am I controlling for these covariates or should I add the
covariates using the WITH function and exclude them from FIXED section?


MIXED depression score BY Time(IV) Suicidal ideation (control) Anxiety
(control)

  /FIXED=Time(IV) Suicidal ideation (control) Anxiety (control) | SSTYPE(3)

  /METHOD=ML

  /PRINT=G  SOLUTION

/RANDOM=INTERCEPT|SUBJECT(consumid) COVTYPE(UN)

  /SAVE=PRED RESID.

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关键词：when With whe use Participants following regarding function question however

First, if it is to be in the model at all it must be in the "Mixed" command.  In the Mixed command use of "by" and "with" depends on the scale of measurement of the predictor.  For a continuous predictor (or control)  use "with" use "by" for categorical variables.  Both with and by can be in the same line.
For example
MIXED depression score BY Time(IV) Suicidal ideation WITH Anxiety

If I understand the second half of your question correctly you also need to add the variable into the fixed effect.  Your analysis will then give the independent contributions of your IV and of the control variable to the outcome.