Linear Mixed Effects
Mixed effects models refer to a variety of models which
have as a key feature both fixed and random effects.
The distinction between fixed and random effects is a murky
one. As pointed out by
Gelman (2005), there are several, often conflicting, definitions of fixed
effects as well as definitions of random effects. Gelman offers a fairly
intuitive solution in the form of renaming fixed effects and random effects and
providing his own clear definitions of each. “We define effects (or
coefficients) in a multilevel model as constant if they are identical for
all groups in a population and varying if they are allowed to differ from
group to group” (Gelman, p. 21). Other ways of thinking about fixed and random
effects, which may be useful but are not always consistent with one another or
those given by Gelman above, are discussed in the next paragraph.
effects are ones in which the possible values of the variable are fixed. Random
effects refer to variables in which the set of potential outcomes can change.
Stated in terms of populations, fixed
effects can be thought of as effects for which the population elements are
fixed. Cases or individuals do not move into or out of the population. Random
effects can be thought of as effects for which the population elements are
changing or can change (i.e. random variable). Cases or individuals can and do
move into and out of the population. Another way of thinking about the
distinction between fixed and random effects is at the observation level. Fixed
effects assume scores or observations are independent while random effects
assume some type of relationship exists between some scores or
observations. For instance, it can be said that gender is a fixed effect
variable because we know all the values of that variable (male & female) and
those values are independent of one another (mutually exclusive); and they
(typically) do not change. A variable such as high school class has random
effects because we can only sample some of the classes which exist; not to
mention, students move into and out of those classes each year.
There are many types of random effects, such as repeated
measures of the same individuals; where the scores at each time of measure
constitute samples from the same participants among a virtually infinite (and
possibly random) number of times of measure from those participants.
Another example of a random effect can be seen in nested designs, where for
example; achievement scores of students are nested within classes and those
classes are nested within schools. That would be an example of a hierarchical
design structure with a random effect for scores nested within classes and a
second random effect for classes nested within schools. The nested data
structure assumes a relationship among groups such that members of a class are
thought to be similar to others in their class in such a way as to distinguish
them from members of other classes and members of a school are thought to be
similar to others in their school in such a way as to distinguish them from
members of other schools. The example used below deals with a similar design
which focuses on multiple fixed effects and a single nested random effect.
2. Linear Mixed Effects
Linear mixed effects models simply model the fixed and
random effects as having a linear form. Similar to the General Linear Model, an
outcome variable is contributed to by additive fixed and random effects (as well
as an error term). Using the familiar notation, the linear mixed effect model
takes the form:
= β1x1ij + β2x2ij … βnxnij
+ bi1z1ij + bi2z2ij …
binznij + εij
where yij is the value of the outcome
variable for a particular ij case, β1 through βn are the
fixed effect coefficients (like regression coefficients), x1ij
through xnij are the fixed effect variables (predictors) for
observation j in group i (usually the first is reserved for the
intercept/constant; x1ij = 1), bi1 through
bin are the random effect coefficients which are assumed to be
multivariate normally distributed, z1ij through znij are
the random effect variables (predictors), and εij is the error for
case j in group i where each group’s error is assumed to be multivariate
3. Example Data
The example used for this tutorial is fictional data where
the interval scaled outcome variable Extroversion (extro) is predicted by fixed
effects for the interval scaled predictor Openness to new experiences (open),
the interval scaled predictor Agreeableness (agree), the interval scaled
predictor Social engagement (social), and the nominal scaled predictor Class (classRC);
as well as the random (nested) effect of Class (classRC) within School (schoolRC)
as well as the random effect of School (schoolRC). The data contains 1200 cases
evenly distributed among 24 nested groups (4 classes within 6 schools). The data
set is available
4. Running the Analysis
Begin by clicking on Analyze, Mixed Models, Linear...
The initial dialogue box is self-explanatory; but will not be used in this
example so click the Continue button.
Next, we have the main Linear Mixed Models dialogue box. Here we specify the
variables we want included in the model. Using the arrows; move extro to the
Dependent Variable box, move classRC and schoolRC to the Factor(s) box, and move
open, agree, and social to the Covariat(s) box. Then click on the Fixed...
button to specify the fixed effects.
The fixed effects in a LINEAR mixed effects model are essentially the same as
a traditional ordinary least squares linear regression. To specify the fixed
effects, use the Add button to move open, agree, social, and classRC into the
Model box. Notice we are not specifying any interaction terms for this model.
Then click the Continue button.
Next, click on the Random... button to specify the random effects.
The first thing we need to do is click on the Build nested terms circle
(marked with the top, centered red ellipse). Then, highlight / select the
classRC factor and use the down arrow button (marked with the lower, left red
ellipse) to move classRC into the Build Term box. Then click the (Within) button
(marked with the lower, middle ellipse). Next, highlight / select the schoolRC
factor and use the down arrow button again to move it inside the parentheses
created by the (Within) button. Next, click the Add button (marked with a red
ellipse inside a green ellipse) to move our nested term into the Model box.
Next, click on the Build terms circle (marked with the green ellipse in the
upper left). Then, highlight / select schoolRC factor and use the Add button
(marked with the green ellipse around the red ellipse) to move schoolRC to the
Model box. Next, click the Continue button at the bottom of the dialogue box.
Next, click on the Estimation... button.
Next, change the Maximum iterations from the default (100) to 150 (marked
with the red soft rectangle). This step is not technically necessary, but it
insures the estimated values match those produced in R using the lme4 package. Then, click the Continue button.
Next, click on the Statistics... button. While some of the options are not
necessary (Case Processing Summary), I generally click all of them.
Next, click on the EM Means... button (Estimated Marginal Means). When the (OVERALL) factor is moved to
the Display Means for box, the grand mean will be produced. The classRC factor
is present (and moved to the Display Means for box) because it is the only
factor (categorical variable) included in the model as a fixed effect. The other
fixed effects are not categorical and thus do not appear here. Next, click the
Next, click on the Save... button. It is generally a good idea to save the
Predicted values. The Fixed Predicted Values will be predicted values based
solely on the Fixed Effects part of the model; while the lower Predicted Values
& Residuals Predicted values will be the whole model's predicted values. Next,
click the Continue button.
Then click the Paste button. Your syntax should match what is below. The
reason I recommend pasting the syntax is that it takes quite a few clicks to
create one of these types of models and it is often the case that multiple
models are run during a session and changing variables or options is simply
easier in the syntax than pointing and clicking back through all the above
Next, highlight / select all the text in the syntax and then click the green
'run' arrow (marked with the red ellipse).
Your output should be the same as what is below.
5. Interpreting the Output.
The Case Processing Summary (above) simply shows that the cases are balanced
among the categories of the categorical variables and no cases were excluded.
The next, rather large table contains all the descriptive statistics (only
the very top of the table is shown here; below).
The Model Dimension table (below) simply shows the model in terms of which
variables (and their number of levels) are fixed and / or random effects and the
number of parameters being estimated.
The next table displays fit indices. For each index; the lower the number,
the better the model fits the data. Generally I use and recommend the Bayesian
Information Criterion (BIC).
The next table contains the results of the Fixed Effects tests; here we see
the intercept and the classRC variables appear to be the main contributors.
The next 5 tables do not offer much information and simply show each
parameter function (only the first and part of the second tables of the five are
The next table "Estimates of Fixed Effects" (below) is very important and shows the
parameter estimates for the Fixed Effects specified in the model. It should be
clear, this table and its interpretation are exactly like one would expect from
a traditional ordinary least squares linear regression. One thing to note is the
way SPSS chooses the reference category for categorical variables. You may have
noticed we have been using the classRC and schoolRC variables instead of the
original class and school variables in the data set. The RC variables contain
the same information as the original variables, they simply have been ReCoded or
Reverse Coded so that the output here will match the output produced using the
lme4 package in the R programming language. It is important to know that SPSS
(and SAS) automatically choose the category with the highest numerical value (or
the lowest alphabetical letter) as the reference category for categorical
variables. All packages I have used in the R programming language choose the
reference category in the more intuitive but opposite way. In the lme4 package
(and others I've used) in R, the software automatically picks the lowest
numerical value (or the earliest alphabetically letter) as the reference
category for categorical variables. This has drastic implications for the
intercept estimate and more troubling, the predicted values produced by a model.
For example, if this same model is specified with the original variables (not
reverse coded) then the Fixed Effects intercept term is 63.049612; so you can
imagine how much different the predicted values would be in that model compared
to this model where the intercept is 57.383879. Recall from multiple regression,
interpreted as the mean of the outcome (extro) when all the predictors have a
value of zero. The predictor estimates (coefficients or slopes) are interpreted
the same way as the coefficients from a traditional regression. For instance, a
one unit increase in the predictor Openness to new experiences (open)
corresponds to a 0.006130 increase in the outcome Extroversion (extro).
Likewise, a one unit increase in the predictor Agreeableness (agree) corresponds
to a 0.007736 decrease in the outcome Extroversion (extro). Furthermore,
the categorical predictor classRC = 3 has a coefficient of 2.054798; which
means, the mean Extroversion score of the third group of classRC (3) is
2.0547978 higher than the mean Extroversion score of the last group of classRC
(4). ClassRC (4) was automatically coded as the reference category.
The next 2 tables simply show the correlation matrix and covariance matrix
for the fixed effects estimates. We can see that multicollinearity is not an
issue among the predictors because, their correlations (and covariances) are
quite low (except of course, the categories of the classRC variable which as
expected, are related).
Next, we have the Estimates of Covariance Parameters table (below); which are
the parameter estimates for the Random Effects. These are variance estimates
(with standard errors, Wald Z test statistics, significance values, and
confidence intervals for the variance estimates). Recall the ubiquitous ANOVA
summary table where we generally have a total variance estimate (sums of
squares) at the bottom, then just above it we have a residual or within groups
variance estimate (sums of squares) and then we have each treatment or between
groups variance estimate (sums of squares). This table is very much like that,
but the total is not displayed and the residual variance estimate is on top. So,
we can quickly calculate the total variance estimate: 95.171929 + 2.883600 +
.968368 = 99.0239 then we can create an R² type of
effect size to gauge the importance of each random effect by dividing the
effect's variance estimate by the total variance estimate to arrive at a
proportion of variance explained or accounted for by each random effect. This is
analogous to an Eta-squared (η²) in standard ANOVA or an R² in regression; it is
sometimes referred to (in the linear mixed effects situation) as an Intraclass
Correlation Coefficient (ICC, Bartko, 1976; Bliese, 2009). For
example, we find that the nested effect of classRC within schoolRC is 2.883600 /
99.0239 = 0.02912024 or simply stated, that random nested effect only accounts
for 2.9% of the variance of the random effects. However,
the random effect for schoolRC alone accounts for 95.171929 / 99.0239 =
0.9611006 or 96% of the variance of the random effects.
If none of the random effects account for a meaningful amount of variance in the
random effects (i.e. if the residual variance is larger than the random effect
variance estimates), then the random effects should be eliminated from the model and a
standard General Linear Model (or Generalized Linear Model) should be fitted
(i.e., a model with only the fixed effects). Notice, SPSS does not
calculate the standard errors correctly and therefore, the confidence interval
estimates and the results of the Wald Z test are NOT valid. The Wald Z
test simply divides the estimate by its standard error to arrive at a Z-score to
test for significance with the standard normal distribution of Z-scores.
However, the standard errors do not match with the standard errors produced when
using the lme4 package in the R programming language. The good news is that the
variance estimates are correct (do match) and the proportion of variance
estimates can be correctly computed and used as effect size measures.
The next two tables simply show the correlation and covariances for the
random effect parameter estimates.
The next three tables in the output are the Random Effects Covariance
Structure matrices. They are omitted here because they are particularly useless
and redundant; because each table simply lists the parameter estimate for each
The last part of the output contains tables with the Estimated Marginal means
(EM means) for the Grand Mean and ClassRC.
The Grand Mean contrast coefficients table and actual grand mean table (the
overall mean of the outcome variable: extro).
The ClassRC variable's contrast coefficients table and mean extroversion (extro)
for each group table.
As with most of the tutorials / pages within this site, this page should not
be considered an exhaustive review of the topic covered and it should not be
considered a substitute for a good textbook.
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