R: Estimated marginal means of linear trends

emtrends {emmeans}

R Documentation

Estimated marginal means of linear trends

Description

The emtrends function is useful when a fitted model involves a numerical predictor x interacting with another predictor a (typically a factor). Such models specify that x has a different trend depending on a; thus, it may be of interest to estimate and compare those trends. Analogous to the emmeans setting, we construct a reference grid of these predicted trends, and then possibly average them over some of the predictors in the grid.

Usage

emtrends(model, specs, var, delta.var = 0.01 * rng, data,
  transform = c("none", "response"), ...)

Arguments

`model`	A supported model object (not a reference grid)
`specs`	Specifications for what marginal trends are desired – as in `emmeans`
`var`	Character value giving the name of a variable with respect to which a difference quotient of the linear predictors is computed. In order for this to be useful, `var` should be a numeric predictor that interacts with at least one factor in `specs`. Then instead of computing EMMs, we compute and compare the slopes of the `var` trend over levels of the specified other predictor(s). As in EMMs, marginal averages are computed for the predictors in `specs` and `by`. See also the “Generalizations” section below.
`delta.var`	The value of h to use in forming the difference quotient (f(x+h) - f(x))/h. Changing it (especially changing its sign) may be necessary to avoid numerical problems such as logs of negative numbers. The default value is 1/100 of the range of `var` over the dataset.
`data`	As in `ref_grid`, you may use this argument to supply the dataset used in fitting the model, for situations where it is not possible to reconstruct the data. Otherwise, leave it missing.
`transform`	If `object` has a response transformation or link function, then specifying `transform = "response"` will cause `emtrends` to calculate the trends after back-transforming to the response scale. This is done using the chain rule, and standard errors are estimated via the delta method. With `transform = "none"` (the default), the trends are calculated on the scale of the linear predictor, without back-transforming it. This argument works similarly to the `transform` argument in `ref_grid`, in that the returned object is re-gridded to the new scale (see also `regrid`).
`...`	Additional arguments passed to other methods or to `ref_grid`

Value

An emmGrid or emm_list object, according to specs. See emmeans for more details on when a list is returned.

Generalizations

Instead of a single predictor, the user may specify some monotone function of one variable, e.g., var = "log(dose)". If so, the chain rule is applied. Note that, in this example, if model contains log(dose) as a predictor, we will be comparing the slopes estimated by that model, whereas specifying var = "dose" would perform a transformation of those slopes, making the predicted trends vary depending on dose.

Examples

fiber.lm <- lm(strength ~ diameter*machine, data=fiber)
# Obtain slopes for each machine ...
( fiber.emt <- emtrends(fiber.lm, "machine", var = "diameter") )
# ... and pairwise comparisons thereof
pairs(fiber.emt)

# Suppose we want trends relative to sqrt(diameter)...
emtrends(fiber.lm, ~ machine | diameter, var = "sqrt(diameter)", 
         at = list(diameter = c(20, 30)))

[Package emmeans version 1.1.3 Index]