Exch {demest} | R Documentation

## Specify an exchangeable prior.

### Description

Specify a prior for main effects or interactions in which the units are exchangeable. Units are exchangeable if the labels or ordering of the units does not provide any information about their distribution.

### Usage

    Exch(covariates = NULL, error = Error())

### Arguments

`covariates` | An object of class `Covariates` or `NULL` (the default). If `covariates` is `NULL`, the prior does not include covariates.
`error` | An object of class `Error`.

### Details

The prior specified by `Exch` has the form

`parameter[j] = covariates[j] + error[j].`

Including a covariates term can make the assumption of exchangeability more plausible. For instance, treating regional effects as exchangeable is more plausible after the region effects have been adjusted for differences in regions' incomes and population sizes. A covariates term can be specified using function `Covariates`.

Errors can have a normal or t distribution. The error term is specified using function `Error`.

### Value

An object of class `SpecExch`.

### See Also

`ExchFixed` specifies a simpler exchangeable prior. `DLM` specifies a non-exchangeable prior.

### Examples

    Exch()
    Exch(error = Error(robust = TRUE))
    Exch(error = Error(scale = HalfT(scale = 0.5)))

    ## with covariates
    data <- data.frame(region = letters[1:10],
                       income = rnorm(10))
    Exch(covariates = Covariates(mean ~ income, data = data))

[Package demest version 0.0.0.3.1 Index]