ExchFixed {demest} | R Documentation

## Specify a simple exchangeable prior.

### Description

A simpler exchangeable prior than `Exch`. The units are assumed to be drawn from a normal distribution with no covariates and a known standard deviation.

### Usage

    ExchFixed(mean = 0, sd = NULL, mult = 1)

### Arguments

`mean` | Mean. Optional.
`sd` | Standard deviation. Optional.
`mult` | Multiplier applied to `sd`, if `sd` is generated automatically. Defaults to 1.

### Details

The prior specified by `ExchFixed` has the form

`parameter[j] ~ N(mean, sd^2).`

`mean` defaults to 0. In most cases, non-zero means are ignored, with a warning, when the model is generated. The exception is when `ExchFixed` is being used to specify an intercept term, when generating simulated data.

If a value for `sd` is not supplied by the user, then a value is generated when function `estimateModel`, `estimateCounts`, or `estimateAccount` is called. Let s be the standard deviation of data `y`, or of `log(y)` in the case of a Poisson model without exposure, and let m be the `mult` argument. The default value for `sd` is then

Model | Default
Poisson with exposure | m
Poisson without exposure | ms
binomial | m
normal | ms.

These value are designed to be weakly informative, in that they favour values for `sd` that are demographically plausible.

`ExchFixed` can be useful a dimension with few elements, such as sex dimension, where there is insufficient information to justify more complicated priors.

### Value

An object of class `SpecExchFixed`.

### See Also

`Exch` specifies a more complicated exchangeable prior.

### Examples

    ## default standard deviation
    ExchFixed()
    ## user-specified standard deviation
    ExchFixed(sd = 0.5)
    ## reduce the size of the automatically-generated 'sd'
    ExchFixed(mult = 0.5)
    ## increase the size of the automatically-generated 'sd'
    ExchFixed(mult = 2)

[Package demest version 0.0.0.3.1 Index]