models {lsmeans} | R Documentation

## Models supported in lsmeans

### Description

Here we document what model objects may be used with lsmeans, and some special features of some of them. We start with those in the stats package; the other packages follow in alphabetical order.

Certain objects are affected by optional arguments to functions that construct `ref.grid` or `lsmobj` objects, including `ref.grid`, `lsmeans`, `lstrends`, and `lsmip`. When “arguments” are mentioned in the subsequent object-by-object documentation, we are talking about arguments in these constructors.

Additional models can be supported by writing appropriate `recover.data` and `lsm.basis` methods. See `extending-lsmeans` and `vignette("extending")` for details.

### stats package

lm, aov, glm

  No extended features. Note that the `lm` support often extends to a number of model objects that inherit from it, such as `rlm` in the MASS package and `rsm` in the rsm package.

mlm, maov, manova

  When there is a multivariate response, the different responses are treated as if they were levels of a factor – named `rep.meas` by default. The `mult.name` argument may be used to change this name. The `mult.levs` argument may specify a named list of one or more sets of levels. If this has more than one element, then the multivariate levels are expressed as combinations of the named factor levels via the function `expand.grid`.

aovlist

  Support for these objects is limited. To avoid strong biases in the predictions, the `contrasts` attribute of all factors should be of a type that sums to zero – for example, `"contr.sum"`, `"contr.poly"`, or `"contr.helmert"` but not `"contr.treatment"`. Only intra-block estimates of covariances are used. That is, if a factor appears in more than one error stratum, only the covariance structure from its lowest stratum is used in estimating standard errors. Degrees of freedom are obtained using the Satterthwaite method. In general, `aovList` support is best with balanced designs, and due caution in the use of contrasts. If a `vcov.` argument is supplied, it must yield a single covariance matrix for the unique fixed effects, and the degrees of freedom are set to `NA`.

### afex package

mixed

  Support for `mixed` objects has been removed. Version 0.14 and later of afex provides new object classes with their own lsmeans support.

### betareg package

betareg

  The additional `mode` argument has possible values of `"response"`, `"link"`, `"precision"`, `"phi.link"`, `"variance"`, and `"quantile"`, which have the same meaning as the `type` argument in `predict.betareg` – with the addition that `"phi.link"` is like `"link"`, but for the precision portion of the model. When `mode = "quantile"` is specified, the additional argument `quantile` (a numeric scalar or vector) specifies which quantile(s) to compute; the default is 0.5 (the median). Also in `"quantile"` mode, an additional variable `quantile` is added to the reference grid, and its levels are the values supplied.

### CARBayes package

carbayes

  The user must supply (via the `data` argument) the dataset used in fitting the model. As with other MCMC-based objects, the summaries and such are frequentist, but the `as.mcmc` method provides a posterior sample of the desired quantities.

### coxme package

coxme

  No additional options. Support for these models is experimental; may throw errors or incorrect results.

### gam package

gam

  Currently, `gam` objects are not supported. Past versions of lsmeans appeared to support `gam` models owing to inheritance from `lm`, but the results were incorrect because spline features were ignored. We now explicitly trap `gam` objects to avoid these misleading analyses.

### gee and geepack packages

These models all have more than one covariance estimate available, and it may be selected by supplying a string as the `vcov.method` argument. It is partially matched with the available choices; thus, for example, vcov = "n" translates to vcov.method = "naive"

gee

  Available covariance estimates are specified in `vcov.method` as `"robust"` (the default) and `"naive"`.

geeglm, geese

  Available covariance estimates are specified in `vcov.method` as `"vbeta"` (the default), `"vbeta.naiv"`, `"vbeta.j1s"`, or `"vbeta.fij"`. The aliases `"robust"` (for `"vbeta"`) and `"naive"` (for `"vbeta.naiv"` are also accepted.

If a matrix or function is supplied as `vcov.method`, it is interpreted as a `vcov.` specification as described for `...` in `ref.grid`.

### glmmADMB package

glmmadmb

  No extended features.

### lme4 package

lmerMod

  There is an optional `mode` argument that defaults to `get.lsm.option("lmer.df")` (which in turn defaults to `"satterthwaite"`). The possible values are "satterthwaite", "kenward-roger", and "asymptotic" (these are partially matched and case-insensitive). With `"satterthwaite"`, d.f. are obtained using code from the lmerTest package, if installed. With `"kenward-roger"`, d.f. are obtained using code from the pbkrtest package, if installed. With `"asymptotic"`, or if the needed package is not installed, d.f. are set to `NA`.

  A by-product of the Kenward-Roger method is that the covariance matrix is adjusted using `vcovAdj`. This can require considerable computation; so to avoid that overhead, the user should opt for the Satterthwaite or asymptotic method; or, for backward compatibility, may disable the use of pbkrtest via lsm.options(disable.pbkrtest=TRUE) (this does not disable the pbkrtest package entirely, just its use in lsmeans). The computation time required depends roughly on the number of observations, N, in the design matrix (because a major part of the computation involves inverting an N x N matrix). Thus, pbkrtest is automatically disabled if N exceeds the value of `get.lsm.option("pbkrtest.limit")`. If desired, the user may use `lsm.options` to adjust this limit from the default of 3000.

  The `df` argument may be used to specify some other degrees of freedom. Note that if `df` and `method = "satterthwaite"` are both specified, the covariance matrix is adjusted but the K-R degrees of freedom are not used.

glmerMod

  No degrees of freedom are available for these objects, so tests and confidence intervals are asymptotic.

### lme4.0 package

mer

  Only asymptotic results are available (no d.f.).

### MASS package

glmmPQL

  Supported by virtue of inheritance from `lme` in the nlme package.

glm.nb

  Supported by virtue of inheritance from `glm`.

polr

  There are two optional arguments: `mode` and `rescale` (which defaults to c(0,1)). For details, see the documentation below regarding the support for the ordinal package, which produces comparable objects (but since `polr` does not support scale models, `mode="scale"` is not supported). Tests and confidence intervals are asymptotic.

rlm

  Supported by virtue of inheritance from `lm`.

### MCMCglmm package

MCMCglmm

  Currently, I have found no way to reconstruct the data based on information in the object; thus, you must provide the dataset via the `data` argument. In addition, the `contrasts` specifications are not recoverable from the object, so the system default must match what was actually used in fitting the model. The usual `summary`, `test`, etc. methods provide frequentist analyses of the results based on the posterior means and covariances. However, an `as.mcmc` method is provided that creates an `mcmc` object that can be summarized or plotted using the coda package. It provides a posterior sample of lsmeans for the given reference grid, based on the posterior sample of the fixed effects from the `MCMCglmm` object.

### MCMCpack package (and perhaps others)

mcmc

  Certain linear-model or mixed-model objects are of class `mcmc`, and contain a sample from the posterior distribution of fixed-effect coefficients. In some cases (e.g., results of `MCMCregress` and `MCMCpoisson`), the object may include a `"call"` attribute that `lsmeans` can use to reconstruct the data and obtain a basis for the least-squares means. If not, a `formula` and `data` argument are provided that may help produce the right results. In addition, the `contrasts` specifications are not recoverable from the object, so the system default must match what was actually used in fitting the model. As for other MCMC-based objects, the summaries and such are frequentist, but the `as.mcmc` method provides a posterior sample of the desired quantities.

### nlme package

gls

  No additional features. Degrees of freedom are computed using `N - p` in `object$dims`. This is consistent with `nlme:::summary.gls` but seems questionable.

lme

  Degrees of freedom are obtained using a containment method, i.e., the minimum of those elements of `object$fixDF$X` receiving nonzero weight (but with a correction to the `lme` object's intercept df). (This is similar to SAS's containment method, but I believe SAS does it incorrectly when the estimands are not contrasts.) The optional argument `sigmaAdjust` (defaults to `TRUE`) will adjust standard errors like in `summary.lme` when the model is fitted using the `"ML"` method. Note: `sigmaAdjust` is comparable to `adjustSigma` in `summary.lme` but it is renamed to avoid conflicting with `adjust`.

nlme

  Support is provided for inferences on parameters named in the `fixed` part of the model. The user must specify `param` in the call and give the name of a parameter that appears in the right-hand side of a `fixed` formula. Degrees of freedom are obtained using the containment-like method described above for `lme`.

### nnet package

multinom

  The reference grid includes a pseudo-factor with the same name and levels as the multinomial response. There is an optional `mode` argument which should match `"prob"` or `"latent"`. With `mode = "prob"`, the reference-grid predictions consist of the estimated multinomial probabilities. The `"latent"` mode returns the linear predictor, recentered so that it averages to zero over the levels of the response variable (similar to sum-to-zero contrasts). Thus each latent variable can be regarded as the log probability at that level minus the average log probability over all levels.

  Please note that, because the probabilities sum to 1 (and the latent values sum to 0) over the multivariate-response levels, all sensible results from `lsmeans` must involve that response as one of the factors. For example, if `resp` is a response with k levels, `lsmeans(model, ~ resp | trt)` will yield the estimated multinomial distribution for each `trt`; but `lsmeans(model, ~ trt)` will just yield the average probability of 1/k for each `trt`.

### ordinal package

clm,clmm

  The reference grid will include all variables that appear in the main model as well as those in the `scale` or `nominal` models. There are two optional arguments: `mode` (a character string) and `rescale` (which defaults to c(0,1)). `mode` should match one of `"latent"` (the default), `"linear.predictor"`, `"cum.prob"`, `"exc.prob"`, `"prob"`, `"mean.class"`, or `"scale"`.

  With mode = "latent", the reference-grid predictions are made on the scale of the latent variable implied by the model. The scale and location of this latent variable are arbitrary, and may be altered via `rescale`. The predictions are multiplied by rescale[2], then rescale[1] is added. Keep in mind that the scaling is related to the link function used in the model; for example, changing from a probit link to a logistic link will inflate the latent values by around pi/sqrt(3), all other things being equal. `rescale` has no effect for other values of `mode`.

  With mode = "linear.predictor" `mode = "cum.prob"`, and `mode = "exc.prob"`, the boundaries between categories (i.e., thresholds) in the ordinal response are included in the reference grid as a pseudo-factor named `cut`. The reference-grid predictions are then of the cumulative probabilities at each threshold (for `mode = "cum.prob"`), exceedance probabilities (one minus cumulative probabilities, for `mode = "exc.prob"`), or the link function thereof (for `mode = "linear.predictor"`).

  With `mode = "prob"`, a pseudo-factor with the same name as the model's response variable is created, and the grid predictions are of the probabilities of each class of the ordinal response. With `"mean.class"`, the returned results are means of the ordinal response, interpreted as a numeric value from 1 to the number of classes, using the `"prob"` results as the estimated probability distribution for each case.

  With `mode = "scale"`, and the fitted object incorporates a scale model, least-squares means are obtained for the factors in the scale model instead of the response model. The grid is constructed using only the factors in the scale model.

  Any grid point that is non-estimable by either the location or the scale model (if present) is set to `NA`, and any LS-means involving such a grid point will also be non-estimable. A consequence of this is that if there is a rank-deficient `scale` model, and then all latent responses become non-estimable because the predictions are made using the average log-scale estimate.

  Tests and confidence intervals are asymptotic.

### pscl package

hurdle, zeroinfl

  Two optional arguments – `mode` and `lin.pred` – are provided. The `mode` argument has possible values `"response"` (the default), `"count"`, `"zero"`, or `"prob0"`. `lin.pred` is logical and defaults to `FALSE`.

  With `lin.pred = FALSE`, the results are comparable to those returned by `predict(..., type = "response")`, `predict(..., type = "count")`, `predict(..., type = "zero")`, or `predict(..., type = "prob")[, 1]`. See the documentation for `predict.hurdle` and `predict.zeroinfl`.

  The option `lin.pred = TRUE` only applies to `mode = "count"` and `mode = "zero"`. The results returned are on the linear-predictor scale, with the same transformation as the link function in that part of the model. The predictions for a reference grid with `mode = "count"`, `lin.pred = TRUE`, and `type = "response"` will be the same as those obtained with `lin.pred = FALSE` and `mode = "count"`; however, any LS means derived from these grids will be different, because the averaging is done on the log-count scale and the actual count scale, respectively – thereby producing geometric means versus arithmetic means of the predictions.

  If the `vcov.` argument is used (see details in `ref.grid`), it must yield a matrix of the same size as would be obtained using `vcov.hurdle` or `vcov.zeroinfl` with its `model` argument set to `("full", "count", "zero")` in respective correspondence with `mode` of `("mean", "count", "zero")`. If `vcov.` is a function, it must support the `model` argument.

### rms package

Potential masking issue

  Both rms and lsmeans offer `contrast` methods, and whichever package is loaded later masks the other. Thus, you may need to call `lsmeans::contrast` or `rms::contrast` explicitly to access the one you want.

Objects inheriting from rms

  Standard support is provided. However, with models having more than one intercept (e.g. from `orm`), a `mode` argument is provided that works similarly to that for the ordinal package. The available modes are `"middle"` (the default), `"latent"`, `"linear.predictor"`, `"cum.prob"`, `"exc.prob"`, `"prob"`, and `"mean.class"`. All are as described for the ordinal package, except as noted below.

  With `mode = "middle"` (this is the default), the middle intercept is used, comparable to the default for rms's `Predict` function. This is quite similar in concept to `mode = "latent"`, where all intercepts are averaged together.

  Results for `mode = "linear.predictor"` are reversed from those in the ordinal package, because `orm` models predict the link function of the upper-tail (exceedance) probabilities.

  With `mode = "prob"`, a pseudo-factor is created having the same name as the model response variable, but its levels are always integers 1, 2, ... regardless of the levels of the original response.

### rstanarm package

stanreg

  Support for models fitted using `stan_xxx` is similar to that for models fitted by `xxx`, where supported, except that posterior samples are also available. For example, `stan_glm` models are treated like `stats::glm`, and `stan_polr` results are similar to those for `MASS::polr` (including its available `mode` and `rescale` options). Models fitted using `stan_biglm`, `stan_betareg`, and `stan_gamm4` are currently not supported. The user may use `as.mcmc`, `as.mcmc.list`, or `as.stanfit` on the `ref.grid` or `lsmobj` produced to obtain posterior samples of LS means, contrasts, etc.

### survival package

survreg, coxph

  No extended features.

### Author(s)

Russell V. Lenth

### See Also

`ref.grid`, `lsm.basis`

[Package lsmeans version 2.27-61 Index]