This vignette describes how `galamm`

can be used to
estimate models with mixed response types.

### Mixed Normal and Binomial Response

We start with the `mresp`

dataset, which comes with the
package. The variable “itemgroup” defines the response type; it equals
“a” for normally distributed responses and “b” for binomially
distributed responses. They are link through a common random
intercept.

```
head(mresp)
#> id x y itemgroup
#> 1 1 0.8638214 0.2866329 a
#> 2 1 0.7676133 2.5647490 a
#> 3 1 0.8812059 1.0000000 b
#> 4 1 0.2239725 1.0000000 b
#> 5 2 0.7215696 -0.4721698 a
#> 6 2 0.6924851 1.1750286 a
```

In terms of the GALAMM defined in the introductory vignette, and for simplicity assuming we use canonical link functions, we have the response model

$f\left(y_{ij} | \nu_{ij}, \phi\right) = \exp \left( \frac{y_{ij}\nu_{ij} - b\left(\nu_{ij}\right)}{\phi} + c\left(y_{ij}, \phi\right) \right)$

for the
$i$th
observation of the
$j$
subject. Although we don’t show this with a subscript, when the variable
`itemgroup = "a"`

we have a Gaussian response, so
$b(\nu) = \nu^{2}/2$
and the support of the distribution is the entire real line
$\mathbb{R}$.
The mean in this case is given by
$\mu_{ij} = \nu_{ij}$.
When `itemgroup = "b"`

we have a binomial response, so
$b(\nu) = \log(1 + \exp(\nu))$
and the support is
$\{0, 1\}$.
In this binomial case we also have
$\phi=1$.
The mean in this case is given by
$\mu_{ij} = \exp(\nu_{ij}) / (1 + \exp(\nu_{ij}))$.
The function
$c(y_{ij}, \phi)$
also differs between these cases, but is not of the same interest, since
it does not depend on the linear predictor. It hence matters for the
value of the log-likelihood, but not for its derivative with respect to
the parameters of interest.

Next, the nonlinear predictor is given by

$\nu_{ij} = \beta_{0} + x_{ij}\beta_{1} + \mathbf{z}_{ij}^{T}\boldsymbol{\lambda} \eta$

where
$x_{ij}$
is an explanatory and
$\mathbf{z}_{ij}$
is a dummy vector of length 2 with exactly one element equal to one and
one element equal to zero. When `itemgroup = "a"`

,
$\mathbf{z} = (1, 0)^{T}$
and when `itemgroup = "b"`

,
$\mathbf{z} = (0, 1)^{T}$.
The parameter
$\boldsymbol{\lambda} = (1, \lambda)^{T}$
is a vector of factor loadings, whose first element equals zero for
identifiability.
$\eta$
is a latent variable, in this case representing an underlying trait
“causing” the observed responses.

The structural model is simply

$\eta = \zeta \sim N(0, \psi),$

where $N(0, \psi)$ denotes a normal distribution with mean 0 and variance $\psi$.

We define the loading matrix as follows, where the value
`1`

indicates that the first element
$\boldsymbol{\lambda}$
is fixed, and the value `NA`

indicates that its second
element is unknown, and to be estimated.

We set `load.var = "itemgroup"`

because all rows of the
data with the same value of `itemgroup`

will receive the same
factor loading. In `formula`

, we state
`(0 + level | id)`

to specify that each subject, identified
by `id`

, has a given level. `level`

is not an
element of the `mresp`

data, but is instead the factor onto
which the loading matrix loads. We identify this with the argument
`factor = "level"`

. We could have chosen any other name for
`"level"`

, except for names that are already columns in
`mresp`

.

We also need to define the response families, with the vector

`families <- c(gaussian, binomial)`

We also need a way of telling galamm which row belongs to which
family, which we do with a family mapping. The values in
`family_mapping`

refer to the elements of
`families`

.

`family_mapping <- ifelse(mresp$itemgroup == "a", 1, 2)`

We are now ready to estimate the mixed response model.

```
mixed_resp <- galamm(
formula = y ~ x + (0 + level | id),
data = mresp,
family = families,
family_mapping = family_mapping,
load.var = "itemgroup",
lambda = loading_matrix,
factor = "level"
)
```

We can also look at its summary output.

```
summary(mixed_resp)
#> GALAMM fit by maximum marginal likelihood.
#> Formula: y ~ x + (0 + level | id)
#> Data: mresp
#>
#> AIC BIC logLik deviance df.resid
#> 9248.7 9280.2 -4619.3 3633.1 3995
#>
#> Lambda:
#> level SE
#> lambda1 1.000 .
#> lambda2 1.095 0.09982
#>
#> Random effects:
#> Groups Name Variance Std.Dev.
#> id level 1.05 1.025
#> Number of obs: 4000, groups: id, 1000
#>
#> Fixed effects:
#> Estimate Std. Error z value Pr(>|z|)
#> (Intercept) 0.041 0.05803 0.7065 4.799e-01
#> x 0.971 0.08594 11.2994 1.321e-29
```

#### Mixed Response and Heteroscedastic Residuals

Mixed response models can be combined with heteroscedastic residuals,
which are described in the vignette on linear
mixed models with heteroscedastic residuals. However, some care is
needed in this case. We illustrate with the dataset
`mresp_hsced`

, whose first few lines are shown below:

```
head(mresp_hsced)
#> id x y itemgroup grp isgauss
#> 1 1 0.8638214 0.2866329 a b 1
#> 2 1 0.7676133 2.5647490 a b 1
#> 3 1 0.8812059 1.0000000 b b 0
#> 4 1 0.2239725 1.0000000 b b 0
#> 5 2 0.7215696 -11.0020148 a a 1
#> 6 2 0.6924851 1.1750286 a b 1
```

This dataset has the same structure as `mresp`

, except
that the residual standard deviation of the normally distributed
responses vary according to the variable `grp`

, which has two
levels. Two fit a model on this dataset with heteroscedastic residuals
for the normally distributed variable, we must use the dummy variable
`isgauss`

when setting up the `weights`

. This is
illustrated below. By writing `~ (0 + isgauss | grp)`

we
specify that the heteroscedasticity is assumed between levels of
`grp`

, but that this only applies when `isgauss`

is nonzero. Hence, the binomially distributed responses don’t have any
heteroscedastic residuals estimated, since for these observations the
dispersion parameter
$\phi$
is fixed to 1. We also ignore the factor loadings for simplicity.

```
family_mapping <- ifelse(mresp$itemgroup == "a", 1L, 2L)
mod <- galamm(
formula = y ~ x + (1 | id),
weights = ~ (0 + isgauss | grp),
family = c(gaussian, binomial),
family_mapping = family_mapping,
data = mresp_hsced
)
```

The summary output now shows that we also have estimated a variance function.

```
summary(mod)
#> GALAMM fit by maximum marginal likelihood.
#> Formula: y ~ x + (1 | id)
#> Data: mresp_hsced
#> Weights: ~(0 + isgauss | grp)
#>
#> AIC BIC logLik deviance df.resid
#> 12356.9 12388.3 -6173.4 31426.4 3995
#>
#> Random effects:
#> Groups Name Variance Std.Dev.
#> id (Intercept) 0 0
#> Number of obs: 4000, groups: id, 1000
#>
#> Variance function:
#> a b
#> 1.00000 0.07756
#>
#> Fixed effects:
#> Estimate Std. Error z value Pr(>|z|)
#> (Intercept) 0.01523 0.06303 0.2416 8.091e-01
#> x 0.99563 0.11153 8.9267 4.388e-19
```

### Covariate Measurement Error Model

This example is taken from Chapter 14.2 in Skrondal and Rabe-Hesketh (2004), and
follows the analyses in (Rabe-Hesketh,
Pickles, and Skrondal 2003; Rabe-Hesketh,
Skrondal, and Pickles 2003). No originality is claimed with
respect to the analyses; but for the sake of understanding the
`galamm`

code, we explain it in quite some detail. The
original dataset comes from Morris, Marr, and
Clayton (1977).

The scientific question concerns the impact of fiber intake on risk
of coronary heat disease. 337 middle aged men were followed, all of whom
either worked as bank staff or as London Transport staff (indicated by
dummy variable `bus`

). All men had a first measurement of
fiber intake, and some had an additional measurement six months later.
In addition, all had a binary variable indicating whether or not they
had developed coronary heart disease.

The first few rows of the dataset are printed below. All of these had only a single measurement of fiber intake.

```
head(diet)
#> id age bus item y chd fiber fiber2
#> 1 1 -0.38 1 fiber1 17.814280 0 1 0
#> 2 1 -0.38 1 chd 0.000000 1 0 0
#> 3 2 0.54 1 fiber1 9.487736 0 1 0
#> 4 2 0.54 1 chd 0.000000 1 0 0
#> 5 3 8.78 1 fiber1 15.958630 0 1 0
#> 6 3 8.78 1 chd 0.000000 1 0 0
```

It is instructive to also look at some men who had two measurements of fiber intake. Here are three of them.

```
diet[diet$id %in% c(219, 220, 221), ]
#> id age bus item y chd fiber fiber2
#> 429 219 -8.13 0 fiber1 15.64263 0 1 0
#> 430 219 -8.13 0 fiber2 14.87973 0 1 1
#> 431 219 -8.13 0 chd 0.00000 1 0 0
#> 432 220 -1.13 0 fiber1 13.46374 0 1 0
#> 433 220 -1.13 0 fiber2 14.73168 0 1 1
#> 434 220 -1.13 0 chd 0.00000 1 0 0
#> 435 221 3.67 0 fiber1 15.95863 0 1 0
#> 436 221 3.67 0 fiber2 17.28778 0 1 1
#> 437 221 3.67 0 chd 0.00000 1 0 0
```

#### Structural Model

Our first goal is to model the effect of fiber intake on risk of heart disease. Since this variable is very likely to be subject to measurement error, it is well known that simply averaging the two measurements, where available, and using the single measurement otherwise, will lead to bias and lack of power (Carroll et al. 2006). Instead, we let $\eta_{j}$ denote the true (latent) fiber intake of person $j$, and define the structural model

$\eta_{j} = \mathbf{x}_{ij}'\boldsymbol{\gamma} + \zeta_{j}.$

where
$\mathbf{x}_{ij}$
is a vector of covariates age, bus, and their interaction, as well as a
constant term for defining the intercept. The R formula for setting up
the model matrix would be
`model.matrix(~ age * bus, data = diet)`

. The term
$\zeta_{j}$
is a normally distributed disturbance,
$\zeta_{j} \sim N(0, \psi)$.

#### Measurement Model

It is assumed that the fiber measurements are normally distributed around the true value $\eta_{j}$, and allow for a drift term $d_{ij}\beta_{0}$, where $d_{2ij}$ is a dummy variable whose value equals one if the $i$th row of the $j$th person is a replicate fiber measurement, and zero otherwise. This allows for a drift in the measurements, and the model becomes

$y_{ij} = d_{2ij} \beta_{0} + \eta_{j} + \epsilon_{ij}, \qquad \epsilon_{ij} = N(0, \theta).$

#### Disease Model

Next, for the rows corresponding to coronary heart disease measurement, we assume a binomial model with a logit link function, i.e., logistic regression. This model is given by

$\text{logit}[P(y_{ij}=1 | \eta_{j})] = \mathbf{x}_{ij}'\boldsymbol{\beta} + \lambda \eta_{j},$

where $\mathbf{x}_{ij}$ is the same as before, but where $\boldsymbol{\beta}$ now represents the direct effect of age and bus on the probability of coronary heart disease. The factor loading $\lambda$ is the regression coefficient for latent fiber intake on the risk of coronary heart disease.

#### Nonlinear Predictor

Stacking the three responses, which are fiber intake at times 1 and 2, and coronary heart disease, we can define the joint model as a GLLAMM with nonlinear predictor

$\nu_{ij} = d_{2ij} \beta_{0} + d_{3ij} \mathbf{x}_{j}'\boldsymbol{\beta} + \mathbf{x}_{j}'\boldsymbol{\gamma}\left[(d_{1i} + d_{2i}) + \lambda d_{3i}\right] + \zeta_{j} \left[(d_{1i} + d_{2i}) + \lambda d_{3i}\right],$

where $d_{1i}$ is a dummy variable for fiber measurements at timepoint 1 and $d_{3i}$ is an indicator for coronary heart disease.

#### Estimating the Model with galamm

In the measurement model for fiber above, note that we implicitly
have factor loadings equal to one. Letting the `item`

variable define which rows should receive which loading, we note that
the factor levels are as follows:

```
levels(diet$item)
#> [1] "fiber1" "fiber2" "chd"
```

We could of course have changed this, but given these levels, we define the following loading matrix:

That is, “fiber1” and “fiber2” receive a loading fixed to one, whereas the loading for “chd” remains to be estimated.

We also need to define the families, making sure it is binomial for “chd” and Gaussian for “fiber1” and “fiber2”.

The model formula also needs some care in this case. We define it as
follows, and note that the whole part
`0 + chd + fiber + fiber2`

could have been replaced by
`item`

. We use the former for ease of explanation.

```
formula <- y ~ 0 + chd + (age * bus):chd + fiber +
(age * bus):fiber + fiber2 + (0 + loading | id)
```

The initial zero specifies that we don’t want R to insert an
intercept term. Next, `chd + (age * bus) : chd`

applies to
“chd” rows only, and corresponds to
$\mathbf{x}_{ij}^{T} \boldsymbol{\beta}$
in the disease model. The part `fiber + (age * bus) : fiber`

corresponds to the term
$\mathbf{x}_{ij}^{T}\boldsymbol{\gamma}$
in the disease model, and `fiber2`

corresponds to the drift
term
$d_{2ij}\beta_{0}$.

We can inspect the model implied by the formula by using the
`nobars`

function from lme4 (Bates et al.
2015) to remove the random effects, and then
`model.matrix`

. It looks correctly set up.

```
head(model.matrix(nobars(formula), data = diet))
#> chd fiber fiber2 chd:age chd:bus age:fiber bus:fiber chd:age:bus age:bus:fiber
#> 1 0 1 0 0.00 0 -0.38 1 0.00 -0.38
#> 2 1 0 0 -0.38 1 0.00 0 -0.38 0.00
#> 3 0 1 0 0.00 0 0.54 1 0.00 0.54
#> 4 1 0 0 0.54 1 0.00 0 0.54 0.00
#> 5 0 1 0 0.00 0 8.78 1 0.00 8.78
#> 6 1 0 0 8.78 1 0.00 0 8.78 0.00
```

Finally, the random effects part `(0 + loading | id)`

specifies that for each `id`

there should be a single random
intercept, which corresponds to latent fiber intake
$\eta_{j}$.
Writing `0 + loading`

means that
$\eta_{j}$
should be multiplied by the factor loadings defined in the loading
matrix.

We are now ready to fit the model.

```
mod <- galamm(
formula = formula,
data = diet,
family = families,
family_mapping = family_mapping,
factor = "loading",
load.var = "item",
lambda = loading_matrix
)
```

Looking at the summary output, however, we see a warning that the Hessian matrix is rank deficient.

```
summary(mod)
#> Warning in vcov.galamm(object, parm = "lambda"): Rank deficient Hessian matrix.Could not compute covariance matrix.
#> Warning in vcov.galamm(object, "beta"): Rank deficient Hessian matrix.Could not compute covariance matrix.
#> GALAMM fit by maximum marginal likelihood.
#> Formula: formula
#> Data: diet
#>
#> AIC BIC logLik deviance df.resid
#> 2837.6 2892.9 -1406.8 12529.3 730
#>
#> Lambda:
#> loading SE
#> lambda1 1.000 .
#> lambda2 1.000 .
#> lambda3 -2.026 .
#>
#> Random effects:
#> Groups Name Variance Std.Dev.
#> id loading 0 0
#> Number of obs: 742, groups: id, 333
#>
#> Fixed effects:
#> Estimate Std. Error z value Pr(>|z|)
#> chd -1.78692 NA NA NA
#> fiber 17.96184 NA NA NA
#> fiber2 -0.64927 NA NA NA
#> chd:age 0.06682 NA NA NA
#> chd:bus -0.06882 NA NA NA
#> fiber:age -0.20480 NA NA NA
#> fiber:bus -1.69601 NA NA NA
#> chd:age:bus -0.04934 NA NA NA
#> fiber:age:bus 0.16097 NA NA NA
```

We suspect that this is a convergence issue of the algorithm, and
start by simply turning a lever to make the algorithm more verbose. The
argument `trace = 3`

is passed onto `optim`

.

```
mod <- galamm(
formula = formula,
data = diet,
family = families,
family_mapping = family_mapping,
factor = "loading",
load.var = "item",
lambda = loading_matrix,
control = galamm_control(optim_control = list(trace = 3))
)
#> N = 11, M = 20 machine precision = 2.22045e-16
#> At X0, 0 variables are exactly at the bounds
#> At iterate 0 f= 2148 |proj g|= 122.68
#> At iterate 10 f = 1770.3 |proj g|= 30.656
#> At iterate 20 f = 1467.2 |proj g|= 11.286
#> At iterate 30 f = 1417.6 |proj g|= 73.904
#> At iterate 40 f = 1406.8 |proj g|= 0.21144
#>
#> iterations 43
#> function evaluations 51
#> segments explored during Cauchy searches 44
#> BFGS updates skipped 0
#> active bounds at final generalized Cauchy point 1
#> norm of the final projected gradient 0.0123301
#> final function value 1406.8
#>
#> F = 1406.8
#> final value 1406.801105
#> converged
```

Now we got some information, but the final estimate is the same,
since we otherwise used the same parameters. We now instead try to
increase the initial estimate of the random effect parameter
$\theta$.
In general
$\theta$
is a vector containing elements of the Cholesky factor of the covariance
matrix (see, Bates et al. (2015)), but in
this case it is just the standard deviation of the latent variable. By
default, its initial value is 1, but we now try to increase it to 10,
with the `start`

argument.

```
mod <- galamm(
formula = formula,
data = diet,
family = families,
family_mapping = family_mapping,
factor = "loading",
load.var = "item",
lambda = loading_matrix,
start = list(theta = 10),
control = galamm_control(optim_control = list(trace = 3))
)
#> N = 11, M = 20 machine precision = 2.22045e-16
#> At X0, 0 variables are exactly at the bounds
#> At iterate 0 f= 2827.6 |proj g|= 123.31
#> At iterate 10 f = 1764.5 |proj g|= 103.86
#> At iterate 20 f = 1623.6 |proj g|= 131.81
#> At iterate 30 f = 1442.4 |proj g|= 35.169
#> At iterate 40 f = 1388 |proj g|= 20.654
#> At iterate 50 f = 1372.4 |proj g|= 1.4042
#>
#> iterations 57
#> function evaluations 69
#> segments explored during Cauchy searches 58
#> BFGS updates skipped 0
#> active bounds at final generalized Cauchy point 0
#> norm of the final projected gradient 0.0119052
#> final function value 1372.16
#>
#> F = 1372.16
#> final value 1372.160387
#> converged
```

The output printed is the negative log-likelihood, and since 1372.16 is less than 1406.8, increasing the initial value to 10 led us to a local optimum with higher likelihood value. Ideally we should have tried other initial values as well, but since Skrondal and Rabe-Hesketh (2004) obtained a negative log-likelihood value of 1372.35 while using adaptive quadrature estimation, we are pretty happy about finding a value close to theirs.

We can now use the summary method, where we find that our estimates are very close to the corresponding model in Table 14.1 of Skrondal and Rabe-Hesketh (2004).

```
summary(mod)
#> GALAMM fit by maximum marginal likelihood.
#> Formula: formula
#> Data: diet
#> Control: galamm_control(optim_control = list(trace = 3))
#>
#> AIC BIC logLik deviance df.resid
#> 2768.3 2823.6 -1372.2 2002.9 730
#>
#> Lambda:
#> loading SE
#> lambda1 1.0000 .
#> lambda2 1.0000 .
#> lambda3 -0.1339 0.05121
#>
#> Random effects:
#> Groups Name Variance Std.Dev.
#> id loading 23.64 4.862
#> Number of obs: 742, groups: id, 333
#>
#> Fixed effects:
#> Estimate Std. Error z value Pr(>|z|)
#> chd -1.91520 0.27229 -7.03361 2.013e-12
#> fiber 17.94834 0.48686 36.86566 1.641e-297
#> fiber2 0.22406 0.41783 0.53624 5.918e-01
#> chd:age 0.06613 0.05931 1.11515 2.648e-01
#> chd:bus -0.02900 0.34355 -0.08441 9.327e-01
#> fiber:age -0.21207 0.10090 -2.10172 3.558e-02
#> fiber:bus -1.68278 0.63721 -2.64084 8.270e-03
#> chd:age:bus -0.04998 0.06507 -0.76812 4.424e-01
#> fiber:age:bus 0.16821 0.11223 1.49882 1.339e-01
```

Now we were able to reproduce results very close to those in Table 14.1 on page 420 of Skrondal and Rabe-Hesketh (2004). This is a useful assurance that the algorithm used by galamm is correctly implemented.

#### Comparison to a Model with Indirect Effect Only

The model above assumed that the age and bus variables had both an indirect effect on the risk of coronary heart disease through its impact on fiber intake, as well as a direct effect. Still following Skrondal and Rabe-Hesketh (2004), we now estimate an alternative model which only has indirect effects. The disease model is now

$\text{logit}[P(y_{ij}=1 | \eta_{j})] = d_{ij3}\beta_{03} + \lambda \eta_{j},$

The formula becomes

```
formula0 <- y ~ 0 + chd + fiber + (age * bus):fiber + fiber2 +
(0 + loading | id)
```

Everything else is the same as before, so we fit the new model with

```
mod0 <- galamm(
formula = formula0,
data = diet,
family = families,
family_mapping = family_mapping,
factor = "loading",
load.var = "item",
lambda = loading_matrix,
start = list(theta = 10),
control = galamm_control(optim_control = list(trace = 3))
)
#> N = 8, M = 20 machine precision = 2.22045e-16
#> At X0, 0 variables are exactly at the bounds
#> At iterate 0 f= 2827.6 |proj g|= 123.31
#> At iterate 10 f = 1664.7 |proj g|= 25.235
#> At iterate 20 f = 1427.9 |proj g|= 37.186
#> At iterate 30 f = 1373.5 |proj g|= 5.1449
#>
#> iterations 37
#> function evaluations 40
#> segments explored during Cauchy searches 38
#> BFGS updates skipped 0
#> active bounds at final generalized Cauchy point 0
#> norm of the final projected gradient 0.000304544
#> final function value 1373.01
#>
#> F = 1373.01
#> final value 1373.013263
#> converged
```

Looking at the summary output, the estimates are now close to the first column in Table 14.1 of Skrondal and Rabe-Hesketh (2004).

```
summary(mod0)
#> GALAMM fit by maximum marginal likelihood.
#> Formula: formula0
#> Data: diet
#> Control: galamm_control(optim_control = list(trace = 3))
#>
#> AIC BIC logLik deviance df.resid
#> 2764.0 2805.5 -1373.0 2007.9 733
#>
#> Lambda:
#> loading SE
#> lambda1 1.000 .
#> lambda2 1.000 .
#> lambda3 -0.136 0.05148
#>
#> Random effects:
#> Groups Name Variance Std.Dev.
#> id loading 23.64 4.862
#> Number of obs: 742, groups: id, 333
#>
#> Fixed effects:
#> Estimate Std. Error z value Pr(>|z|)
#> chd -1.9668 0.18248 -10.7783 4.360e-27
#> fiber 17.9741 0.48123 37.3504 2.502e-305
#> fiber2 0.2238 0.41779 0.5358 5.921e-01
#> fiber:age -0.1896 0.09914 -1.9122 5.585e-02
#> fiber:bus -1.6991 0.62529 -2.7173 6.581e-03
#> fiber:age:bus 0.1515 0.11012 1.3760 1.688e-01
```

We can use `anova`

method for galamm objects to compare
the models. AIC and BIC favor the simpler model with only indirect
effects.

```
anova(mod, mod0)
#> Data: diet
#> Models:
#> mod0: formula0
#> mod: formula
#> npar AIC BIC logLik deviance Chisq Df Pr(>Chisq)
#> mod0 9 2764.0 2805.5 -1373.0 2002.9
#> mod 12 2768.3 2823.6 -1372.2 2002.9 1.7058 3 0.6357
```

## References

*Journal of Statistical Software*67 (1): 1–48. https://doi.org/10.18637/jss.v067.i01.

*Measurement Error in Nonlinear Models: A Modern Perspective*. 2nd ed. Monographs on Statistics and Applied Probability. Boca Raton, Florida: John Wiley & Sons.

*Br Med J*2 (6098): 1307–14. https://doi.org/10.1136/bmj.2.6098.1307.

*Statistical Modelling*3 (3): 215–32. https://doi.org/10.1191/1471082X03st056oa.

*The Stata Journal*3 (4): 386–411. https://doi.org/10.1177/1536867X0400300408.

*Generalized Latent Variable Modeling*. Interdisciplinary Statistics Series. Boca Raton, Florida: Chapman and Hall/CRC.