R: Linear and Non-linear formulas in 'brms'

brmsformula-helpers {brms} | R Documentation

## Linear and Non-linear formulas in brms

### Description

Helper functions to specify linear and non-linear formulas for use with `brmsformula`.

### Usage

    nlf(formula, ..., flist = NULL, dpar = NULL, resp = NULL, loop = NULL)

    lf(
      ...,
      flist = NULL,
      dpar = NULL,
      resp = NULL,
      center = NULL,
      cmc = NULL,
      sparse = NULL,
      decomp = NULL
    )

    acformula(autocor, resp = NULL)

    set_nl(nl = TRUE, dpar = NULL, resp = NULL)

    set_rescor(rescor = TRUE)

    set_mecor(mecor = TRUE)

### Arguments

`formula` | Non-linear formula for a distributional parameter. The name of the distributional parameter can either be specified on the left-hand side of `formula` or via argument `dpar`.
`...` | Additional `formula` objects to specify predictors of non-linear and distributional parameters. Formulas can either be named directly or contain names on their left-hand side. Alternatively, it is possible to fix parameters to certain values by passing numbers or character strings in which case arguments have to be named to provide the parameter names. See 'Details' for more information.
`flist` | Optional list of formulas, which are treated in the same way as formulas passed via the `...` argument.
`dpar` | Optional character string specifying the distributional parameter to which the formulas passed via `...` and `flist` belong.
`resp` | Optional character string specifying the response variable to which the formulas passed via `...` and `flist` belong. Only relevant in multivariate models.
`loop` | Logical; Only used in non-linear models. Indicates if the computation of the non-linear formula should be done inside (`TRUE`) or outside (`FALSE`) a loop over observations. Defaults to `TRUE`.
`center` | Logical; Indicates if the population-level design matrix should be centered, which usually increases sampling efficiency. See the 'Details' section for more information. Defaults to `TRUE` for distributional parameters and to `FALSE` for non-linear parameters.
`cmc` | Logical; Indicates whether automatic cell-mean coding should be enabled when removing the intercept by adding `0` to the right-hand of model formulas. Defaults to `TRUE` to mirror the behavior of standard R formula parsing.
`sparse` | Logical; indicates whether the population-level design matrices should be treated as sparse (defaults to `FALSE`). For design matrices with many zeros, this can considerably reduce required memory. Sampling speed is currently not improved or even slightly decreased.
`decomp` | Optional name of the decomposition used for the population-level design matrix. Defaults to `NULL` that is no decomposition. Other options currently available are `"QR"` for the QR decomposition that helps in fitting models with highly correlated predictors.
`autocor` | A one sided formula containing autocorrelation terms. All none autocorrelation terms in `autocor` will be silently ignored.
`nl` | Logical; Indicates whether `formula` should be treated as specifying a non-linear model. By default, `formula` is treated as an ordinary linear model formula.
`rescor` | Logical; Indicates if residual correlation between the response variables should be modeled. Currently this is only possible in multivariate `gaussian` and `student` models. Only relevant in multivariate models.
`mecor` | Logical; Indicates if correlations between latent variables defined by `me` terms should be modeled. Defaults to `TRUE`.

### Value

For `lf` and `nlf` a `list` that can be passed to `brmsformula` or added to an existing `brmsformula` or `mvbrmsformula` object. For `set_nl` and `set_rescor` a logical value that can be added to an existing `brmsformula` or `mvbrmsformula` object.

### See Also

`brmsformula`, `mvbrmsformula`

### Examples

    # add more formulas to the model
    bf(y ~ 1) +
      nlf(sigma ~ a * exp(b * x)) +
      lf(a ~ x, b ~ z + (1|g)) +
      gaussian()

    # specify 'nl' later on
    bf(y ~ a * inv_logit(x * b)) +
      lf(a + b ~ z) +
      set_nl(TRUE)

    # specify a multivariate model
    bf(y1 ~ x + (1|g)) +
      bf(y2 ~ z) +
      set_rescor(TRUE)

    # add autocorrelation terms
    bf(y ~ x) + acformula(~ arma(p = 1, q = 1) + car(W))

[Package brms version 2.17.0 Index]