View source: R/estimate_slopes.R
estimate_slopes | R Documentation |
Estimate the slopes (i.e., the coefficient) of a predictor over or within different factor levels, or alongside a numeric variable. In other words, to assess the effect of a predictor at specific configurations data. It corresponds to the derivative and can be useful to understand where a predictor has a significant role when interactions or non-linear relationships are present.
Other related functions based on marginal estimations includes
estimate_contrasts()
and estimate_means()
.
See the Details section below, and don't forget to also check out the Vignettes and README examples for various examples, tutorials and use cases.
estimate_slopes(
model,
trend = NULL,
by = NULL,
ci = 0.95,
p_adjust = "none",
transform = NULL,
keep_iterations = FALSE,
backend = getOption("modelbased_backend", "marginaleffects"),
verbose = TRUE,
...
)
model |
A statistical model. |
trend |
A character indicating the name of the variable for which to compute the slopes. |
by |
The (focal) predictor variable(s) at which to evaluate the desired
effect / mean / contrasts. Other predictors of the model that are not
included here will be collapsed and "averaged" over (the effect will be
estimated across them). |
ci |
Confidence Interval (CI) level. Default to |
p_adjust |
The p-values adjustment method for frequentist multiple
comparisons. For |
transform |
A function applied to predictions and confidence intervals
to (back-) transform results, which can be useful in case the regression
model has a transformed response variable (e.g., |
keep_iterations |
If |
backend |
Whether to use You can set a default backend via |
verbose |
Use |
... |
Other arguments passed, for instance, to
|
The estimate_slopes()
, estimate_means()
and estimate_contrasts()
functions are forming a group, as they are all based on marginal
estimations (estimations based on a model). All three are built on the
emmeans or marginaleffects package (depending on the backend
argument), so reading its documentation (for instance emmeans::emmeans()
,
emmeans::emtrends()
or this website) is
recommended to understand the idea behind these types of procedures.
Model-based predictions is the basis for all that follows. Indeed,
the first thing to understand is how models can be used to make predictions
(see estimate_link()
). This corresponds to the predicted response (or
"outcome variable") given specific predictor values of the predictors (i.e.,
given a specific data configuration). This is why the concept of reference grid()
is so important for direct predictions.
Marginal "means", obtained via estimate_means()
, are an extension
of such predictions, allowing to "average" (collapse) some of the predictors,
to obtain the average response value at a specific predictors configuration.
This is typically used when some of the predictors of interest are factors.
Indeed, the parameters of the model will usually give you the intercept value
and then the "effect" of each factor level (how different it is from the
intercept). Marginal means can be used to directly give you the mean value of
the response variable at all the levels of a factor. Moreover, it can also be
used to control, or average over predictors, which is useful in the case of
multiple predictors with or without interactions.
Marginal contrasts, obtained via estimate_contrasts()
, are
themselves at extension of marginal means, in that they allow to investigate
the difference (i.e., the contrast) between the marginal means. This is,
again, often used to get all pairwise differences between all levels of a
factor. It works also for continuous predictors, for instance one could also
be interested in whether the difference at two extremes of a continuous
predictor is significant.
Finally, marginal effects, obtained via estimate_slopes()
, are
different in that their focus is not values on the response variable, but the
model's parameters. The idea is to assess the effect of a predictor at a
specific configuration of the other predictors. This is relevant in the case
of interactions or non-linear relationships, when the effect of a predictor
variable changes depending on the other predictors. Moreover, these effects
can also be "averaged" over other predictors, to get for instance the
"general trend" of a predictor over different factor levels.
Example: Let's imagine the following model lm(y ~ condition * x)
where
condition
is a factor with 3 levels A, B and C and x
a continuous
variable (like age for example). One idea is to see how this model performs,
and compare the actual response y to the one predicted by the model (using
estimate_expectation()
). Another idea is evaluate the average mean at each of
the condition's levels (using estimate_means()
), which can be useful to
visualize them. Another possibility is to evaluate the difference between
these levels (using estimate_contrasts()
). Finally, one could also estimate
the effect of x averaged over all conditions, or instead within each
condition (using [estimate_slopes]
).
A data.frame of class estimate_slopes
.
library(ggplot2)
# Get an idea of the data
ggplot(iris, aes(x = Petal.Length, y = Sepal.Width)) +
geom_point(aes(color = Species)) +
geom_smooth(color = "black", se = FALSE) +
geom_smooth(aes(color = Species), linetype = "dotted", se = FALSE) +
geom_smooth(aes(color = Species), method = "lm", se = FALSE)
# Model it
model <- lm(Sepal.Width ~ Species * Petal.Length, data = iris)
# Compute the marginal effect of Petal.Length at each level of Species
slopes <- estimate_slopes(model, trend = "Petal.Length", by = "Species")
slopes
## Not run:
# Plot it
plot(slopes)
standardize(slopes)
model <- mgcv::gam(Sepal.Width ~ s(Petal.Length), data = iris)
slopes <- estimate_slopes(model, by = "Petal.Length", length = 50)
summary(slopes)
plot(slopes)
model <- mgcv::gam(Sepal.Width ~ s(Petal.Length, by = Species), data = iris)
slopes <- estimate_slopes(model,
trend = "Petal.Length",
by = c("Petal.Length", "Species"), length = 20
)
summary(slopes)
plot(slopes)
## End(Not run)
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