2
$\begingroup$

Thank you for any help. I am looking at the interaction of time with plasma biomarkers in the brain. Here is the code that I ran:

    test = lmer(precuneus.dvr ~ Cage*time + sex*time + race_binary*time + NFLz*time + (1|idno), 
                data = testDFlong30, 
                na.action = na.omit)

precuneus.dvr: is measured in this unit called DVR,

race: 0 = White and 1 = non-White,

sex: 0 = Female and 1 = male,

time: time between visits for each participant for a brain scan.

My output is here in the image below. How would I interpret the results I see and especially the ones with a time interaction?enter image description here

Improve this question
$\endgroup$
2
  • $\begingroup$ What values can Cage and NFLz take? $\endgroup$
    – Doctor Milt
    Commented Mar 27, 2023 at 14:45
  • $\begingroup$ Please refrain from making substantial edits to a post after it's received an answer, because doing so destroys the work of people who have taken the time to understand & answer your question. If you still have outstanding questions, then you can ask a new Question. $\endgroup$
    – Sycorax ♦
    Commented Mar 28, 2023 at 18:23

1 Answer 1

Reset to default
6
$\begingroup$

Start with time and sex. Ignoring the other variables for now, your line of best fit is

precuneus.dvr = 1.148 + 0.00222 * time + 0.03073 * sex + 0.00417 * time * sex + ...

The more time between visits, the higher the predicted value of precuneus.dvr, but the line for female participants (sex = 0) is

1.148 + 0.00222 * time

whereas for male participants (sex = 1) it is

(1.148 + 0.03073) + (0.00222 + 0.00417) * time

i.e. the two lines have different intercepts and different slopes. Males start from a higher level and the line is steeper (bigger effect as time increases).

You can interpret the interaction of race_binary and time in a similar way.

The variables Cage and NFLz are not binary, so we interpret their interaction with time a little differently, but the idea is the same. For example, the older the participant, the steeper the slope of time.

Centering the variable the way you did (so that Cage = 0 corresponds to average) is a sensible idea, as we can then interpret the value of 2.224e-02 as the slope of time for a participant of average age.

Improve this answer
$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.