Dear statalister,
I am trying to fit a mixed random effect model for a
clinical trial data in
which the treatment was given once and the outcome was
measured at monthly
interval for one year (repeated measure). Age, gender,
post_treatment time
and pre-treatment measurement (coded as 0=low and
1=high) are my independent
variables. I am interested to know whether response
varies between persons
and also over time (i.e. heterogenity in response
between subjects over
time). The Table blow shows that a model with all
fixed effects along with
a random intercept and slope for 'time' (model 4) was
found to be a better
model than a model with all fixed effects but without
random slope (model 3,
based on likelihood ratio test). But all the
coeffcients for the fixed part
as well as random part in model 4 are not
significantly different from zero
(based on z-test). However, with model 3, the
coefficients for some of the
fixed effects (time, mf-density group*time) and the
random intercept are
significantly different from zero. My questions are:
(1) If parameter estimates are not significant but the
LR test suggests that
model 4 provides significantly better fit than model
3, which one of the two
models (model 3 and model 4) is the most parsimonious?
(2) on what basis (LR test or z-test for the parameter
estimates)?
Estimates of logistic regressions with and without
random coefficients for
days post-treatment
Covariate Model 1 Model 2 Model 3 Model 4
Estimate SE Z Estimate SE Z Estimate SE Z
Estimate SE Z
Fixed part: main effects
Constant -0.7078 0.6126 -1.16 -1.5036 1.6155
-0.93 -1.8591
1.3468 -1.38 -1.4780 2.1583 -0.68
Age -0.0536 0.0202 -2.65 -0.0689 0.0536
-1.29 -0.0656
0.0492 -1.33 -0.1258 0.0813 -1.55
Sex (0=male; 1=Female) 0.0798 0.5977 0.13
-0.2241 1.3824 -0.16 -0.7612
0.9792 -0.78 -0.9868 1.5981 -0.62
Mf-density group: (0=low, 1=High)
High -0.5180 0.7140 -0.73 -1.2402 1.5679 -0.79
Time 0.0039 0.0020 1.97 0.0061 0.0026
2.34 0.0055 0.0019
2.89 0.0054 0.0068 0.79
Fixed part: interactions:
Mf-density group � Time -0.0077 0.0045 -1.72
-0.0112
-2.01 -0.0132 0.0050 -2.62 -0.0234 0.0117
-1.99
Mf-density group � Female -1.3282 1.2179 -1.09
-0.7486 2.3540 -0.32
Female � Time -0.0019 0.0028 -0.67 -0.0031
0.0037 -0.84
Random part
Variance of intercept (1) 5.4469 2.7907
5.3062 2.5835 11.0471
7.5482
Variance of slope for time (2) 0.0003
0.0003
Covariance of (2, 1) -0.0222 0.0235
r (intra class correlation) 0.6233 0.6171
0.7704
Log-likelihood -106.6 -87.4 -88.8
-81.9
thanking you in advance
Subramanian Swaminathan
Vector Control Research Centre
(Indian Council of Medical Research)
Indira Nagar
Pondicherry - 605 006
INDIA
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