require("emmeans")
knitr::opts_chunk$set(fig.width = 4.5, class.output = "ro")

Contents {#contents}

This vignette describes various ways of summarizing emmGrid objects.

  1. summary(), confint(), and test()
  2. Back-transforming to response scale (See also the "transformations" vignette)
  3. Multiplicity adjustments
  4. Using "by" variables
  5. Joint (omnibus) tests
  6. Testing equivalence, noninferiority, nonsuperiority
  7. Graphics (in "basics" vignette)

Index of all vignette topics

summary(), confint(), and test() {#summary}

The most important method for emmGrid objects is summary(). For one thing, it is called by default when you display an emmeans() result. The summary() function has a lot of options, and the detailed documentation via help("summary.emmGrid") is worth a look.

For ongoing illustrations, let's re-create some of the objects in the "basics" vignette for the pigs example:

mod4 <- lm(inverse(conc) ~ source + factor(percent), data = pigs)
RG <- ref_grid(mod4)
EMM.source <- emmeans(RG, "source")

Just summary(<object>) by itself will produce a summary that varies somewhat according to context. It does this by setting different defaults for the infer argument, which consists of two logical values, specifying confidence intervals and tests, respectively. [The exception is models fitted using MCMC methods, where summary() is diverted to the hpd.summary() function, a preferable summary for many Bayesians.]

The summary of a newly made reference grid will show just estimates and standard errors, but not confidence intervals or tests (that is, infer = c(FALSE, FALSE)). The summary of an emmeans() result, as we see above, will have intervals, but no tests (i.e., infer = c(TRUE, FALSE)); and the result of a contrast() call (see comparisons and contrasts) will show test statistics and P values, but not intervals (i.e., infer = c(FALSE, TRUE)). There are courtesy methods confint() and test() that just call summary() with the appropriate infer setting; for example,

test(EMM.source)

It is not particularly useful, though, to test these EMMs against the default of zero -- which is why tests are not usually shown. It makes a lot more sense to test them against some target concentration, say 40. And suppose we want to do a one-sided test to see if the concentration is greater than 40. Remembering that the response is inverse-transformed in this model, and that the inverse transformation reverses the direction of comparisons, so that a right-tailed test on the conc scale becomes a left-tailed test on the inverse(conc) scale,

test(EMM.source, null = inverse(40), side = "<")

It is also possible to add calculated columns to the summary, via the calc argument. The calculations can include any columns up through df in the summary, as well as any variable in the object's grid slot. Among the latter are usually weights in a column named .wgt., and we can use that to include sample size in the summary:

confint(EMM.source, calc = c(n = ~.wgt.))

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Back-transforming {#tran}

Transformations and link functions are supported in several ways in emmeans, making this a complex topic worthy of its own vignette. Here, we show just the most basic approach. Namely, specifying the argument type = "response" will cause the displayed results to be back-transformed to the response scale, when a transformation or link function is incorporated in the model. For example, let's try the preceding test() call again:

test(EMM.source, null = inverse(40), side = "<", type = "response")

Note what changes and what doesn't change. In the test() call, we still use the 1/40 as the null value; null must always be specified on the linear-prediction scale, in this case the inverse. In the output, the displayed estimates, as well as the null value, are shown back-transformed. As well, the standard errors are altered (using the delta method). However, the t ratios and P values are identical to the preceding results. That is, the tests themselves are still conducted on the linear-predictor scale (as is noted in the output).

Similar statements apply to confidence intervals on the response scale:

confint(EMM.source, side = "<", level = .90, type = "response")

With side = "<", an upper confidence limit is computed on the inverse scale, then that limit is back-transformed to the response scale; and since inverse reverses everything, those upper confidence limits become lower ones on the response scale. (We have also illustrated how to change the confidence level.)

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Multiplicity adjustments {#adjust}

Both tests and confidence intervals may be adjusted for simultaneous inference. Such adjustments ensure that the confidence coefficient for a whole set of intervals is at least the specified level, or to control for multiplicity in a whole family of tests. This is done via the adjust argument. For ref_grid() and emmeans() results, the default is adjust = "none". For most contrast() results, adjust is often something else, depending on what type of contrasts are created. For example, pairwise comparisons default to adjust = "tukey", i.e., the Tukey HSD method. The summary() function sometimes changes adjust if it is inappropriate. For example, with

confint(EMM.source, adjust = "tukey")

the adjustment is changed to the Sidak method because the Tukey adjustment is inappropriate unless you are doing pairwise comparisons.

# {#adjmore}

An adjustment method that is usually appropriate is Bonferroni; however, it can be quite conservative. Using adjust = "mvt" is the closest to being the "exact" all-around method "single-step" method, as it uses the multivariate t distribution (and the mvtnorm package) with the same covariance structure as the estimates to determine the adjustment. However, this comes at high computational expense as the computations are done using simulation techniques. For a large set of tests (and especially confidence intervals), the computational lag becomes noticeable if not intolerable.

For tests, adjust increases the P values over those otherwise obtained with adjust = "none". Compare the following adjusted tests with the unadjusted ones previously computed.

test(EMM.source, null = inverse(40), side = "<", adjust = "bonferroni")

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"By" variables {#byvars}

Sometimes you want to break a summary down into smaller pieces; for this purpose, the by argument in summary() is useful. For example,

confint(RG, by = "source")

If there is also an adjust in force when by variables are used, by default, the adjustment is made separately on each by group; e.g., in the above, we would be adjusting for sets of 4 intervals, not all 12 together (but see "cross-adjustments" below.)

There can be a by specification in emmeans() (or equivalently, a | in the formula); and if so, it is passed on to summary() and used unless overridden by another by. Here are examples, not run:

emmeans(mod4, ~ percent | source)     ### same results as above
summary(.Last.value, by = "percent")       ### grouped the other way

Specifying by = NULL will remove all grouping.

Adjustments across by groups {#cross-adjust}

As was mentioned, each by group is regarded as a separate family with regards to the adjust procedure. For example, consider a model with interaction for the warpbreaks data, and construct pairwise comparisons of tension by wool:

warp.lm <- lm(breaks ~ wool * tension, data = warpbreaks)
warp.pw <- pairs(emmeans(warp.lm, ~ tension | wool))
warp.pw

We have two sets of 3 comparisons, and the (default) Tukey adjustment is made separately in each group.

However, sometimes we want the multiplicity adjustment to be broader. This broadening can be done in two ways. One is to remove the by variable, which then treats all results as one family. In our example:

test(warp.pw, by = NULL, adjust = "bonferroni")

This accomplishes the goal of putting all the comparisons in one family of 6 comparisons. Note that the Tukey adjustment may not be used here because we no longer have one set of pairwise comparisons.

An alternative is to specify cross.adjust, which specifies an additional adjustment method to apply to corresponding sets of within-group adjusted P values:

test(warp.pw, adjust = "tukey", cross.adjust = "bonferroni")

These adjustments are less conservative than the previous result, but it is still a conservative adjustment to the set of 6 tests. Had we also specified adjust = "bonferroni", we would have obtained the same adjusted P values as we obtained with by = NULL.

Simple comparisons {#simple}

There is also a simple argument for contrast() that is in essence the inverse of by; the contrasts are run using everything except the specified variables as by variables. To illustrate, let's consider the model for pigs that includes the interaction (so that the levels of one factor compare differently at levels of the other factor).

mod5 <- lm(inverse(conc) ~ source * factor(percent), data = pigs)
RG5 <- ref_grid(mod5)
contrast(RG5, "consec", simple = "percent")

In fact, we may do all one-factor comparisons by specifying simple = "each". This typically produces a lot of output, so use it with care.

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Joint tests {#joint}

From the above, we already know how to test individual results. For pairwise comparisons (details in the "comparisons" vignette), we might do

PRS.source <- pairs(EMM.source)
PRS.source

But suppose we want an omnibus test that all these comparisons are zero. Easy enough, using the joint argument in test (note: the joint argument is not available in summary(); only in test()):

test(PRS.source, joint = TRUE)

Notice that there are three comparisons, but only 2 d.f. for the test, as cautioned in the message.

The test produced with joint = TRUE is a "type III" test (assuming the default equal weights are used to obtain the EMMs). See more on these types of tests for higher-order effects in the "interactions" vignette section on contrasts.

# {#joint_tests}

For convenience, there is also a joint_tests() function that performs joint tests of contrasts among each term in a model or emmGrid object.

joint_tests(RG5)

The tests of main effects are of families of contrasts; those for interaction effects are for interaction contrasts. These results are essentially the same as a "Type-III ANOVA", but may differ in situations where there are empty cells or other non-estimability issues, or if generalizations are present such as unequal weighting. (Another distinction is that sums of squares and mean squares are not shown; that is because these really are tests of contrasts among predictions, and they may or may not correspond to model sums of squares.)

One may use by variables with joint_tests. For example:

joint_tests(RG5, by = "source")

In some models, it is possible to specify submodel = "type2", thereby obtaining something akin to a Type II analysis of variance. See the messy-data vignette for an example.

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Testing equivalence, noninferiority, and nonsuperiority {#equiv}

The delta argument in summary() or test() allows the user to specify a threshold value to use in a test of equivalence, noninferiority, or nonsuperiority. An equivalence test is kind of a backwards significance test, where small P values are associated with small differences relative to a specified threshold value delta. The help page for summary.emmGrid gives the details of these tests. Suppose in the present example, we consider two sources to be equivalent if they are within 0.005 of each other. We can test this as follows:

test(PRS.source, delta = 0.005, adjust = "none")

Using the 0.005 threshold, the P value is quite small for comparing soy and skim, providing some statistical evidence that their difference is enough smaller than the threshold to consider them equivalent.

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Graphics {#graphics}

Graphical displays of emmGrid objects are described in the "basics" vignette

Index of all vignette topics



rvlenth/emmeans documentation built on April 9, 2024, 5:19 a.m.