trinormal {VGAM} | R Documentation

## Trivariate Normal Distribution Family Function

### Description

Maximum likelihood estimation of the nine parameters of a trivariate normal distribution.

### Usage

    trinormal(zero = c("sd", "rho"), eq.mean = FALSE,
        eq.sd = FALSE, eq.cor = FALSE,
        lmean1 = "identitylink", lmean2 = "identitylink",
        lmean3 = "identitylink",
        lsd1   = "loglink", lsd2   = "loglink", lsd3   = "loglink",
        lrho12 = "rhobitlink", lrho23 = "rhobitlink", lrho13 = "rhobitlink",
        imean1 = NULL, imean2 = NULL, imean3 = NULL,
        isd1   = NULL, isd2   = NULL, isd3   = NULL,
        irho12 = NULL, irho23 = NULL, irho13 = NULL, imethod = 1)

### Arguments

`lmean1, lmean2, lmean3, lsd1, lsd2, lsd3` | Link functions applied to the means and standard deviations. See `Links` for more choices. Being positive quantities, a log link is the default for the standard deviations.
`lrho12, lrho23, lrho13` | Link functions applied to the correlation parameters. See `Links` for more choices. By default the correlation parameters are allowed to have a value between -1 and 1, but that may be problematic when `eq.cor = TRUE` because they should have a value between -0.5 and 1.
`imean1, imean2, imean3, isd1, isd2, isd3` | See `CommonVGAMffArguments` for more information.
`irho12, irho23, irho13, imethod, zero` | See `CommonVGAMffArguments` for more information.
`eq.mean, eq.sd, eq.cor` | Logical. Constrain the means or the standard deviations or correlation parameters to be equal?

### Details

For the trivariate normal distribution, this fits a linear model (LM) to the means, and by default, the other parameters are intercept-only. The response should be a three-column matrix. The three correlation parameters are prefixed by `rho`, and the default gives them values between -1 and 1 however, this may be problematic when the correlation parameters are constrained to be equal, etc.. The fitted means are returned as the fitted values, which is in the form of a three-column matrix. Fisher scoring is implemented.

### Value

An object of class `"vglmff"` (see `vglmff-class`). The object is used by modelling functions such as `vglm`, and `vgam`.

### Warning

The default parameterization does not make the estimated variance-covariance matrix positive-definite. In order for the variance-covariance matrix to be positive-definite the quantity `1 - rho12^2 - rho13^2 - rho23^2 + 2 * rho12 * rho13 * rho23` must be positive, and if `eq.cor = TRUE` then this means that the `rho`s must be between -0.5 and 1.

### Author(s)

T. W. Yee

### See Also

`uninormal`, `binormal`, `rtrinorm`.

### Examples

    set.seed(123); nn <- 1000
    tdata <- data.frame(x2 = runif(nn), x3 = runif(nn))
    tdata <- transform(tdata, y1 = rnorm(nn, 1 + 2 * x2),
                              y2 = rnorm(nn, 3 + 4 * x2),
                              y3 = rnorm(nn, 4 + 5 * x2))
    fit1 <- vglm(cbind(y1, y2, y3) ~ x2, data = tdata,
                 trinormal(eq.sd = TRUE, eq.cor = TRUE), trace = TRUE)
    coef(fit1, matrix = TRUE)
    constraints(fit1)
    summary(fit1)
    ## Not run:  # Try this when eq.sd = TRUE, eq.cor = TRUE:
    fit2 <- vglm(cbind(y1, y2, y3) ~ x2, data = tdata, stepsize = 0.25,
                 trinormal(eq.sd = TRUE, eq.cor = TRUE,
                           lrho12 = extlogitlink(min = -0.5),
                           lrho23 = extlogitlink(min = -0.5),
                           lrho13 = extlogitlink(min = -0.5)), trace = TRUE)
    coef(fit2, matrix = TRUE)

    ## End(Not run)

[Package VGAM version 1.1-1 Index]