[R-sig-ME] Should I include participants with baseline score only (missing afterwards) in a longitudinal study?

[K Imran M]

Hi everyone,

I did a longitudinal study where I collected functional score at 3
different times (baseline, 1 month after baseline and 3 months after
baseline) from 98 patients. There were 11 patients who died right after
baseline (so they have functional score at baseline only, and they did not
have the scores at 1 month after baseline or 3 months after baseline).

My question is should I remove 11 patients from the dataset (because they
only provide 1 score?)

What I did was, next , I run the nlme::lme function on 2 datasets, the
first dataset that contained 98 participants (11 with only 1 score at
baseline) and the second dataset with participants with at least 2 scores
(baseline + 1 month or baseline + 3 month or baseline + 1 month + 3 month).
I noticed the lme estimates for the two datasets are slightly different.
How can I explain this?

In the analysis above, I used a random intercept model (participants as the
random effect) with time (baseline, 1 month after baseline and 3 months
after baseline) treated as a factor variable. The covariate is age.

The datasets (edited due to privacy) are from this links:
dat.a (https://drive.google.com/open?id=1jAAFnrUfuTsVQST7EE3vjrh0_71ziAut)
dat.b (https://drive.google.com/open?id=1caGTd6SNnzbHSln84jw9b_lVHhnz7Qij)

And the R codes are here:
#######
library(haven)
dat.a <- read_dta("test_complete_data.dta")
dat.b <- read_dta("test_complete_with_at_discharge.dta")

# mixed model
library(nlme)
mod.dta.a <- lme(barthel ~ -1 + age + factor(time), random = ~1| id,
                          data = dat.a, na.action = 'na.omit', method =
'ML')
mod.dta.b <- lme(barthel ~ -1 + age + factor(time), random = ~1| id,
                 data = dat.b, na.action = 'na.omit', method = 'ML')

# res
summary(mod.dta.a)
summary(mod.dta.b)
#####


So let me rephrase the questions (Let us assume we are not interested in
the mechanism of missingness  but purely on the estimation from mixed model)
1) should I include patients that have only 1 measurement in a longitudinal
study in my model?
2) why the estimates are different from the dataset with at least 2 data on
follow-ups) vs the dataset that also contain participants with only 1 data
on follow-up? A simple explanation should be fine for me.

I apologize for my lack of math and stat skill. I really appreciate your
time in responding to this question.

Thank you.

Best wishes

Kamarul Imran
Universiti Sains Malaysia

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