I am analyzing some longitudinal data (3 time points) where participants were randomly assigned to either a treatment or control condition. Within the treatment condition, participants were randomly assigned to 1 of 5 facilitators. Therefore, I have measurements nested within participants for the control participants and measurements nested within participants nested within facilitator for the treatment participants. How can I fit model that accommodates for this design in lmer() or lme()? If I let, y = response variable trt = 1 for treatment, 0 for control ID = unique identifier for each participant time = days since start of study (3 time points / participant) facilitator = unique identifier for each facilitator, control participants will have NA by design. I would like to fit a model that looks like this with lmer() syntax, lmer(y ~ trt * time + (1 | facilitator) + (1 | facilitator:ID), data = data) But this model (obviously) drops all my treatment participants as they don't have values for facilitator in my current coding. Therefore, I've been fitting this model: m0 <- lmer(y ~ trt * time + (1 | ID) , data = data) But I was searching on StackOverflow and found this link, https://stackoverflow.com/questions/37201754/three-level-partially-nested-model/37205167#37205167, where it's suggested to recode NA for facilitator to "none". When I recode facilitator from NA to "none" and fit this model: m1 <- lmer(y ~ trt * time + (1 | facilitator) + (1 | facilitator:ID), data = data) I get similar estimates to m0, however, the SE increase and the df decrease (using lmerTest) substantially. I am wondering if this is caused by any potential confounding of trt and facilitator because all participants that received the treatment (trt = 1), will have "none" as their facilitator despite not sharing a facilitator. Otherwise, the two models are nearly identical in terms of log-likelihood. I am wondering what approach makes the most sense for my design. Thanks for the help. Chris
Fitting a partially nested model with lmer or lme
1 message · Christopher David Desjardins