## Documentation Center |

` results = compare(lme,altlme)` returns
the results of a likelihood
ratio test that compares the linear mixed-effects models

`compare` tests the following null and alternate
hypotheses:

*H*_{0}: Observed response
vector is generated by `lme`.

*H*_{1}: Observed response
vector is generated by model `altlme`.

It is recommended that you fit `lme` and `altlme` using
the maximum likelihood (ML) method prior to model comparison. If you
use the restricted maximum likelihood (REML) method, then both models
must have the same fixed-effects design matrix.

To test for fixed effects, use `compare` with
the simulated
likelihood ratio test when `lme` and `altlme` are
fit using ML or use the `fixedEffects`, `anova`,
or `coefTest` methods.

` results = compare(___,Name,Value)` also
returns the results of a likelihood ratio test that compares linear
mixed-effects models

For example, you can check if the first input model is nested in the second input model.

`[ results,siminfo]
= compare(lme,altlme,'NSim',nsim)` returns
the results of a simulated likelihood ratio test that compares linear
mixed-effects models

You can fit `lme` and `altlme` using
ML or REML. Also, `lme` does not have to be nested
in `altlme`. If you use the restricted maximum likelihood
(REML) method to fit the models, then both models must have the same
fixed-effects design matrix.

`[ results,siminfo]
= compare(___,Name,Value)` also
returns the results of a simulated likelihood ratio test that compares
linear mixed-effects models

For example, you can change the options for performing the simulated
likelihood ratio test, or change the confidence level of the confidence
interval for the *p*-value.

Under the null hypothesis *H*_{0},
the observed likelihood ratio test statistic has an approximate chi-squared
reference distribution with degrees of freedom `deltaDF`.
When comparing two models, `compare` computes the *p*-value
for the likelihood ratio test by comparing the observed likelihood
ratio test statistic with this chi-squared reference distribution.

The *p*-values obtained using the likelihood
ratio test can be conservative when testing for the presence or absence
of random-effects terms and anticonservative when testing for the
presence or absence of fixed-effects terms. Hence, use the `fixedEffects`, `anova`,
or `coefTest` method or the simulated likelihood
ratio test while testing for fixed effects.

To perform the simulated likelihood ratio test, `compare` first
generates the reference distribution of the likelihood ratio test
statistic under the null hypothesis. Then, it assesses the statistical
significance of the alternate model by comparing the observed likelihood
ratio test statistic to this reference distribution.

`compare` produces the simulated reference
distribution of the likelihood ratio test statistic under the null
hypothesis as follows:

Generate random data

`ysim`from the fitted model`lme`.Fit the model specified in

`lme`and alternate model`altlme`to the simulated data`ysim`.Calculate the likelihood ratio test statistic using results from step 2 and store the value.

Repeat step 1 to 3

`nsim`times.

Then, `compare` computes the *p*-value
for the simulated likelihood ratio test by comparing the observed
likelihood ratio test statistic with the simulated reference distribution.
The *p*-value estimate is the ratio of the number
of times the simulated likelihood ratio test statistic is equal to
or exceeds the observed value plus one, to the number of replications
plus one.

Suppose the observed likelihood ratio statistic is *T*,
and the simulated reference distribution is stored in vector *T*_{H0}.
Then,

To account for the uncertainty in the simulated reference distribution, `compare` computes
a 100*(1 – α)% confidence interval for the true *p*-value.

You can use the simulated likelihood ratio test to compare arbitrary
linear mixed-effects models. That is, when you are using the simulated
likelihood ratio test, `lme` does not have to be
nested within `altlme`, and you can fit `lme` and `altlme` using
either maximum likelihood (ML) or restricted maximum likelihood (REML)
methods. If you use the restricted maximum likelihood (REML) method
to fit the models, then both models must have the same fixed-effects
design matrix.

The `'CheckNesting','True'` name-value pair
argument checks the following requirements.

For a simulated likelihood ratio test:

You must use the same method to fit both models (

`lme`and`altlme`).`compare`cannot compare a model fit using ML to a model fit using REML.You must fit both models to the same response vector.

If you use REML to fit

`lme`and`altlme`, then both models must have the same fixed-effects design matrix.The maximized log likelihood or restricted log likelihood of the bigger model (

`altlme`) must be greater than or equal to that of the smaller model (`lme`).

For a theoretical test, `'CheckNesting','True'` checks
all the requirements listed for a simulated likelihood ratio test
and the following:

Weight vectors you use to fit

`lme`and`altlme`must be identical.If you use ML to fit

`lme`and`altlme`, the fixed-effects design matrix of the bigger model (`altlme`) must contain that of the smaller model (`lme`).The random-effects design matrix of the bigger model (

`altlme`) must contain that of the smaller model (`lme`).

Akaike information criterion (AIC) is *AIC* =
–2*log*L*_{M} +
2*(*nc* + *p* + 1), where log*L*_{M} is
the maximized log likelihood (or maximized restricted log likelihood)
of the model, and *nc* + *p* + 1
is the number of parameters estimated in the model. *p* is
the number of fixed-effects coefficients, and *nc* is
the total number of parameters in the random-effects covariance excluding
the residual variance.

Bayesian information criterion (BIC) is *BIC* =
–2*log*L*_{M} +
ln(*n _{eff}*)*(

If the fitting method is maximum likelihood (ML), then

*n*=_{eff}*n*, where*n*is the number of observations.If the fitting method is restricted maximum likelihood (REML), then

*n*=_{eff}*n*–*p*.

A lower value of deviance indicates a better fit. As the value
of deviance decreases, both AIC and BIC tend to decrease. Both AIC
and BIC also include penalty terms based on the number of parameters
estimated, *p*. So, when the number of parameters
increase, the values of AIC and BIC tend to increase as well. When
comparing different models, the model with the lowest AIC or BIC value
is considered as the best fitting model.

`LinearMixedModel` computes the deviance of
model *M* as minus two times the loglikelihood of
that model. Let *L*_{M} denote
the maximum value of the likelihood function for model *M*.
Then, the deviance of model *M* is

A lower value of deviance indicates a better fit. Suppose *M*_{1} and *M*_{2} are
two different models, where *M*_{1} is
nested in *M*_{2}. Then, the
fit of the models can be assessed by comparing the deviances *Dev*_{1} and *Dev*_{2} of
these models. The difference of the deviances is

Usually, the asymptotic distribution of this difference has
a chi-square distribution with degrees of freedom *v* equal
to the number of parameters that are estimated in one model but fixed
(typically at 0) in the other. That is, it is equal to the difference
in the number of parameters estimated in M_{1} and
M_{2}. You can get the *p*-value
for this test using `1 – chi2cdf(Dev,V)`,
where *Dev* = *Dev*_{2} – *Dev*_{1}.

However, in mixed-effects models, when some variance components fall on the boundary of the parameter space, the asymptotic distribution of this difference is more complicated. For example, consider the hypotheses

*H*_{0}:
*D* is a *q*-by-*q* symmetric
positive semidefinite matrix.

*H*_{1}: *D* is
a (*q*+1)-by-(*q*+1) symmetric positive
semidefinite matrix.

That is, *H*_{1} states
that the last row and column of *D* are different
from zero. Here, the bigger model *M*_{2} has *q* +
1 parameters and the smaller model *M*_{1} has *q* parameters.
And *Dev* has a 50:50 mixture of *χ*^{2}_{q} and *χ*^{2}_{(q +
1)} distributions (Stram and Lee, 1994).

[1] Hox, J. *Multilevel Analysis, Techniques and
Applications*. Lawrence Erlbaum Associates, Inc., 2002.

[2] Stram D. O. and J. W. Lee. "Variance components
testing in the longitudinal mixed-effects model". *Biometrics*,
Vol. 50, 4, 1994, pp. 1171–1177.

`anova` | `covarianceParameters` | `fitlme` | `fitlmematrix` | `fixedEffects` | `LinearMixedModel` | `randomEffects`

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