hurdle {pscl} | R Documentation

## Hurdle Models for Count Data Regression

### Description

Fit hurdle regression models for count data via maximum likelihood.

### Usage

    hurdle(formula, data, subset, na.action, weights, offset,
      dist = c("poisson", "negbin", "geometric"),
      zero.dist = c("binomial", "poisson", "negbin", "geometric"),
      link = c("logit", "probit", "cloglog", "cauchit", "log"),
      control = hurdle.control(...),
      model = TRUE, y = TRUE, x = FALSE, ...)

### Arguments

`formula` | symbolic description of the model, see details.
`data, subset, na.action` | arguments controlling formula processing via `model.frame`.
`weights` | optional numeric vector of weights.
`offset` | optional numeric vector with an a priori known component to be included in the linear predictor of the count model. See below for more information on offsets.
`dist` | character specification of count model family.
`zero.dist` | character specification of the zero hurdle model family.
`link` | character specification of link function in the binomial zero hurdle (only used if `zero.dist = "binomial"`.
`control` | a list of control arguments specified via `hurdle.control`.
`model, y, x` | logicals. If `TRUE` the corresponding components of the fit (model frame, response, model matrix) are returned.
`...` | arguments passed to `hurdle.control` in the default setup.

### Details

Hurdle count models are two-component models with a truncated count component for positive counts and a hurdle component that models the zero counts. Thus, unlike zero-inflation models, there are not two sources of zeros: the count model is only employed if the hurdle for modeling the occurence of zeros is exceeded. The count model is typically a truncated Poisson or negative binomial regression (with log link). The geometric distribution is a special case of the negative binomial with size parameter equal to 1. For modeling the hurdle (occurence of positive counts) either a binomial model can be employed or a censored count distribution. Binomial logit and censored geometric models as the hurdle part both lead to the same likelihood function and thus to the same coefficient estimates. A censored negative binomial model for the zero hurdle is only identified if there is at least one non-constant regressor with (true) coefficient different from zero (and if all coefficients are close to zero the model can be poorly conditioned).

The `formula` can be used to specify both components of the model: If a `formula` of type `y ~ x1 + x2` is supplied, then the same regressors are employed in both components. This is equivalent to `y ~ x1 + x2 | x1 + x2`. Of course, a different set of regressors could be specified for the zero hurdle component, e.g., `y ~ x1 + x2 | z1 + z2 + z3` giving the count data model `y ~ x1 + x2` conditional on (`|`) the zero hurdle model `y ~ z1 + z2 + z3`.

Offsets can be specified in both parts of the model pertaining to count and zero hurdle model: `y ~ x1 + offset(x2) | z1 + z2 + offset(z3)`, where `x2` is used as an offset (i.e., with coefficient fixed to 1) in the count part and `z3` analogously in the zero hurdle part. By the rule stated above `y ~ x1 + offset(x2)` is expanded to `y ~ x1 + offset(x2) | x1 + offset(x2)`. Instead of using the `offset()` wrapper within the `formula`, the `offset` argument can also be employed which sets an offset only for the count model. Thus, `formula = y ~ x1` and `offset = x2` is equivalent to `formula = y ~ x1 + offset(x2) | x1`.

All parameters are estimated by maximum likelihood using `optim`, with control options set in `hurdle.control`. Starting values can be supplied, otherwise they are estimated by `glm.fit` (the default). By default, the two components of the model are estimated separately using two `optim` calls. Standard errors are derived numerically using the Hessian matrix returned by `optim`. See `hurdle.control` for details.

The returned fitted model object is of class `"hurdle"` and is similar to fitted `"glm"` objects. For elements such as `"coefficients"` or `"terms"` a list is returned with elements for the zero and count components, respectively. For details see below.

A set of standard extractor functions for fitted model objects is available for objects of class `"hurdle"`, including methods to the generic functions `print`, `summary`, `coef`, `vcov`, `logLik`, `residuals`, `predict`, `fitted`, `terms`, `model.matrix`. See `predict.hurdle` for more details on all methods.

### Value

An object of class `"hurdle"`, i.e., a list with components including

`coefficients` | a list with elements `"count"` and `"zero"` containing the coefficients from the respective models,
`residuals` | a vector of raw residuals (observed - fitted),
`fitted.values` | a vector of fitted means,
`optim` | a list (of lists) with the output(s) from the `optim` call(s) for minimizing the negative log-likelihood(s),
`control` | the control arguments passed to the `optim` call,
`start` | the starting values for the parameters passed to the `optim` call(s),
`weights` | the case weights used,
`offset` | a list with elements `"count"` and `"zero"` containing the offset vectors (if any) from the respective models,
`n` | number of observations (with weights > 0),
`df.null` | residual degrees of freedom for the null model (= `n - 2`),
`df.residual` | residual degrees of freedom for fitted model,
`terms` | a list with elements `"count"`, `"zero"` and `"full"` containing the terms objects for the respective models,
`theta` | estimate of the additional theta parameter of the negative binomial model(s) (if negative binomial component is used),
`SE.logtheta` | standard error(s) for log(theta),
`loglik` | log-likelihood of the fitted model,
`vcov` | covariance matrix of all coefficients in the model (derived from the Hessian of the `optim` output(s)),
`dist` | a list with elements `"count"` and `"zero"` with character strings describing the respective distributions used,
`link` | character string describing the link if a binomial zero hurdle model is used,
`linkinv` | the inverse link function corresponding to `link`,
`converged` | logical indicating successful convergence of `optim`,
`call` | the original function call,
`formula` | the original formula,
`levels` | levels of the categorical regressors,
`contrasts` | a list with elements `"count"` and `"zero"` containing the contrasts corresponding to `levels` from the respective models,
`model` | the full model frame (if `model = TRUE`),
`y` | the response count vector (if `y = TRUE`),
`x` | a list with elements `"count"` and `"zero"` containing the model matrices from the respective models (if `x = TRUE`).

### Author(s)

Achim Zeileis <Achim.Zeileis@R-project.org>

### References

Cameron, A. Colin and Pravin K. Trivedi. 1998. Regression Analysis of Count Data. New York: Cambridge University Press.

Cameron, A. Colin and Pravin K. Trivedi 2005. Microeconometrics: Methods and Applications. Cambridge: Cambridge University Press.

Mullahy, J. 1986. Specification and Testing of Some Modified Count Data Models. Journal of Econometrics. 33:341–365.

Zeileis, Achim, Christian Kleiber and Simon Jackman 2008. “Regression Models for Count Data in R.” Journal of Statistical Software, 27(8). URL http://www.jstatsoft.org/v27/i08/.

### See Also

`hurdle.control`, `glm`, `glm.fit`, `glm.nb`, `zeroinfl`

### Examples

    ## data
    data("bioChemists", package = "pscl")

    ## logit-poisson
    ## "art ~ ." is the same as "art ~ . | .", i.e.
    ## "art ~ fem + mar + kid5 + phd + ment | fem + mar + kid5 + phd + ment"
    fm_hp1 <- hurdle(art ~ ., data = bioChemists)
    summary(fm_hp1)

    ## geometric-poisson
    fm_hp2 <- hurdle(art ~ ., data = bioChemists, zero = "geometric")
    summary(fm_hp2)

    ## logit and geometric model are equivalent
    coef(fm_hp1, model = "zero") - coef(fm_hp2, model = "zero")

    ## logit-negbin
    fm_hnb1 <- hurdle(art ~ ., data = bioChemists, dist = "negbin")
    summary(fm_hnb1)

    ## negbin-negbin
    ## (poorly conditioned zero hurdle, note the standard errors)
    fm_hnb2 <- hurdle(art ~ ., data = bioChemists, dist = "negbin", zero = "negbin")
    summary(fm_hnb2)

[Package pscl version 1.5.2 Index]