[R-sig-ME] Downdated VtV error for two level mixed model

[Viechtbauer, Wolfgang (SP)]

Hi Matt,

You have already received some answers to your previous post:

https://stat.ethz.ch/pipermail/r-sig-mixed-models/2020q3/028806.html
https://stat.ethz.ch/pipermail/r-sig-mixed-models/2020q3/028807.html
https://stat.ethz.ch/pipermail/r-sig-mixed-models/2020q3/028808.html

Best,
Wolfgang

>-----Original Message-----
>From: R-sig-mixed-models [mailto:r-sig-mixed-models-bounces using r-project.org]
>On Behalf Of Matthew Boden
>Sent: Wednesday, 08 July, 2020 19:36
>To: r-sig-mixed-models using r-project.org
>Subject: [R-sig-ME] Downdated VtV error for two level mixed model
>
>Good afternoon,
>
>I am looking for advice regarding a mixed model I am trying to implement
>using lme4. My two-level random-effects model won’t run, perhaps due to one
>or two issues.
>
>Level 1 are patients clustered in healthcare facilities (‘Station’). The
>outcome is a continuous variable (‘PopCov’) that is calculated at the
>facility-level, and thus a level 2 variable that does not vary at the
>patient level.
>
>My aim is to examine the prediction of PopCov by (a) patient-level (e.g.,
>race/ethnicity, age, symptom severity), and (b) facility-level variables
>(e.g., overall racial/ethnic composition, average age). It is important to
>examine race/ethnicity at both patient and facility-levels because patients
>with different racial/ethnic backgrounds tend to differ in terms of age,
>symptom severity, etc.
>
>Each record/row in my data set is a patient, with facility-level variables
>(including PopCov) having identical values among patients within a given
>facility.
>
>An error is thrown when I run a basic model.
>
>A1 <-lmer(PopCov ~ (1 | Station), data = DISP)
>
>*Error in fn9nM$xeval()) : Downdated VtV is not positive definite
>
>I obtain the same error when I add to the model either a patient-level or
>facility level predictor.
>
>An internet search suggested that I have complete separation of my data
>and/or poorly scaled variables.
>
>I assume this issue has to do with the fact that the outcome is a level 2
>variable. Perhaps compounding the issue is the large and unbalanced nature
>of the data. I have ~6 million patients clustered in ~1000 healthcare
>facilities. Individual facilities have anywhere from 100 to 30000 patients
>clustered in them.
>
>I could use some advice regarding how to specify the model to predict a
>facility-level variable (level 2) from both patient (level 1) and
>facility-level (level 2) variables with these data.
>
>Thank you in advance.
>
>Matt