This page attempts to summarize some of the common problems with
fitting `[gn]lmer`

models and how to troubleshoot them.

Most of the symptoms/diagnoses/workarounds listed below are due to
various issues in the actual mixed model fitting process. You may
run into problems due to multicollinearity or variables that are
incorrectly typed (e.g. a variable is accidentally coded as character
or factor rather than numeric). These problems can often be isolated
by trying a `lm`

or `glm`

fit or attempting to construct the
design matrix via `model.matrix()`

(in each case with the random
effects in your model excluded). If these tests fail then the problem
is likely not specifically an `lme4`

issue.

`failure to converge in (xxxx) evaluations`

The optimizer hit its maximum limit of function evaluations. To increase this, use the`optControl`

argument of`[g]lmerControl`

-- for`Nelder_Mead`

and`bobyqa`

the relevant parameter is`maxfun`

; for`optim`

and`optimx`

-wrapped optimizers, including`nlminbwrap`

, it's`maxit`

; for`nloptwrap`

, it's`maxeval`

.`Model failed to converge with max|grad| ...`

The scaled gradient at the fitted (RE)ML estimates is worryingly large. Tryrefitting the parameters starting at the current estimates: getting consistent results (with no warning) suggests a false positive

switching optimizers: getting consistent results suggests there is not really a problem; getting a similar log-likelihood with different parameter estimates suggests that the parameters are poorly determined (possibly the result of a misspecified or overfitted model)

compute values of the deviance in the neighbourhood of the estimated parameters to double-check that

`lme4`

has really found a local optimum.

`Hessian is numerically singular: parameters are not uniquely determined`

The Hessian (inverse curvature matrix) at the maximum likelihood or REML estimates has a very large eigenvalue, indicating that (within numerical tolerances) the surface is completely flat in some direction. The model may be misspecified, or extremely badly scaled (see`"Model is nearly unidentifiable"`

).`Model is nearly unidentifiable ... Rescale variables?`

The Hessian (inverse curvature matrix) at the maximum likelihood or REML estimates has a large eigenvalue, indicating that the surface is nearly flat in some direction. Consider centering and/or scaling continuous predictor variables.`Contrasts can be applied only to factors with 2 or more levels`

One or more of the categorical predictors in the model has fewer than two levels. This may be due to user error when converting these predictors to factors prior to modeling, or it may result from some factor levels being eliminated due to`NA`

s in other predictors. Double-check the number of data points in each factor level to see which one is the culprit:`lapply(na.omit(df[,vars]), table)`

(where`df`

is the`data.frame`

and`vars`

are the column names of your predictor variables).