hdeffsev {VGAM} | R Documentation

## Hauck-Donner Effects: Severity Measures

### Description

Computes the severity of the Hauck-Donner effect for each regression coefficient of a VGLM regression.

### Usage

    hdeffsev(x, y, dy, ddy, allofit = FALSE, tol0 = 0.1,
             severity.table = c("None", "Faint", "Weak", "Moderate",
                                "Strong", "Extreme", "Undetermined"))

### Arguments

`x, y` | Numeric vectors; `x` are the estimates, and `y` are the Wald statistics.
`dy, ddy` | Numeric vectors; the first and second derivatives of the Wald statistics. They can be computed by `hdeff`.
`allofit` | Logical. If `TRUE` then other quantities are returned in a list. The default is a vector with elements selected from the argument `severity.table`.
`severity.table` | Character vector with 7 values. The last value is used for initialization. Usually users should not assign anything to arguments `severity.table` or `tol0`.
`tol0` | Numeric. Any estimate whose absolute value is less than `tol0` is assigned the first value of the argument `severity.table`, i.e., none. This is to handle a singularity at the origin: the estimates might be extremely close to 0.

### Details

This function is rough-and-ready. It is possible to use the first two derivatives obtained from `hdeff` to categorize the severity of the the Hauck-Donner effect (HDE). It is effectively assumed that, starting at the origin and going right, the curve is made up of a convex segment followed by a concave segment and then the convex segment. Midway in the concave segment the derivative is 0, and beyond that the HDE is really manifest because the derivative is negative.

For `"none"` the estimate lies on the convex part of the curve near the origin, hence there is no HDE at all.

For `"faint"` and `"weak"` the estimate lies on the concave part of the curve but the Wald statistic is still increasing as estimate gets away from 0, hence it is only a mild HDE.

For `"moderate"`, `"strong"` and `"extreme"` the Wald statistic is decreasing as the estimate gets away from 0, hence it really does exhibit the HDE. It is recommended that `lrt.stat` be used to compute LRT p-values, as they do not suffer from the HDE.

### Value

By default this function returns a labelled vector with elements selected from `severity.table`. If `allofit = TRUE` then Yee (2018) gives details about the other list components: a quantity called `zeta` is the normal line projected onto the x-axis, and its first derivative gives additional information about the position of the estimate along the curve.

### Note

This function is likely to change in the short future because it is experimental and far from complete. Improvements are intended.

### Author(s)

Thomas W. Yee.

### See Also

`seglines`, `hdeff`.

### Examples

    deg <- 4  # myfun is a function that approximates the HDE
    myfun <- function(x, deriv = 0) switch(as.character(deriv),
      '0' = x^deg * exp(-x),
      '1' = (deg * x^(deg-1) - x^deg) * exp(-x),
      '2' = (deg * (deg-1) * x^(deg-2) - 2*deg * x^(deg-1) + x^deg) * exp(-x))

    xgrid <- seq(0, 10, length = 101)
    ansm <- hdeffsev(xgrid, myfun(xgrid), myfun(xgrid, deriv = 1),
                     myfun(xgrid, deriv = 2), allofit = TRUE)
    digg <- 4
    cbind(severity = ansm$sev,
          fun      = round(myfun(xgrid), digg),
          deriv1   = round(myfun(xgrid, deriv = 1), digg),
          deriv2   = round(myfun(xgrid, deriv = 2), digg),
          zderiv1  = round(1 + (myfun(xgrid, deriv = 1))^2 +
                           myfun(xgrid, deriv = 2) * myfun(xgrid), digg))

[Package VGAM version 1.1-1 Index]