loglaplace {VGAM} | R Documentation

## Log-Laplace and Logit-Laplace Distribution Family Functions

### Description

Maximum likelihood estimation of the 1-parameter log-Laplace and the 1-parameter logit-Laplace distributions. These may be used for quantile regression for counts and proportions respectively.

### Usage

    loglaplace1(tau = NULL, llocation = "loglink",
        ilocation = NULL, kappa = sqrt(tau/(1 - tau)), Scale.arg = 1,
        ishrinkage = 0.95, parallel.locat = FALSE, digt = 4,
        idf.mu = 3, rep0 = 0.5, minquantile = 0, maxquantile = Inf,
        imethod = 1, zero = NULL)
    logitlaplace1(tau = NULL, llocation = "logitlink",
        ilocation = NULL, kappa = sqrt(tau/(1 - tau)),
        Scale.arg = 1, ishrinkage = 0.95, parallel.locat = FALSE,
        digt = 4, idf.mu = 3, rep01 = 0.5, imethod = 1, zero = NULL)

### Arguments

`tau, kappa` | See `alaplace1`.
`llocation` | Character. Parameter link functions for location parameter xi. See `Links` for more choices. However, this argument should be left unchanged with count data because it restricts the quantiles to be positive. With proportions data `llocation` can be assigned a link such as `logitlink`, `probitlink`, `clogloglink`, etc.
`ilocation` | Optional initial values. If given, it must be numeric and values are recycled to the appropriate length. The default is to choose the value internally.
`parallel.locat` | Logical. Should the quantiles be parallel on the transformed scale (argument `llocation`)? Assigning this argument to `TRUE` circumvents the seriously embarrassing quantile crossing problem.

`imethod` | Initialization method. Either the value 1, 2, or ....
`idf.mu, ishrinkage, Scale.arg, digt, zero` | See `alaplace1`.

`rep0, rep01`

Numeric, positive. Replacement values for 0s and 1s respectively. For count data, values of the response whose value is 0 are replaced by `rep0`; it avoids computing `log(0)`. For proportions data values of the response whose value is 0 or 1 are replaced by `min(rangey01[1]/2, rep01/w[y< = 0])` and `max((1 + rangey01[2])/2, 1-rep01/w[y >= 1])` respectively; e.g., it avoids computing `logitlink(0)` or `logitlink(1)`. Here, `rangey01` is the 2-vector `range(y[(y > 0) & (y < 1)])` of the response.

`minquantile, maxquantile` | Numeric. The minimum and maximum values possible in the quantiles. These argument are effectively ignored by default since `loglink` keeps all quantiles positive. However, if `llocation = logofflink(offset = 1)` then it is possible that the fitted quantiles have value 0 because `minquantile = 0`.

### Details

These VGAM family functions implement translations of the asymmetric Laplace distribution (ALD). The resulting variants may be suitable for quantile regression for count data or sample proportions. For example, a log link applied to count data is assumed to follow an ALD. Another example is a logit link applied to proportions data so as to follow an ALD. A positive random variable Y is said to have a log-Laplace distribution if Y = exp(W) where W has an ALD. There are many variants of ALDs and the one used here is described in `alaplace1`.

### Value

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

In the `extra` slot of the fitted object are some list components which are useful. For example, the sample proportion of values which are less than the fitted quantile curves, which is `sum(wprior[y <= location]) / sum(wprior)` internally. Here, `wprior` are the prior weights (called `ssize` below), `y` is the response and `location` is a fitted quantile curve. This definition comes about naturally from the transformed ALD data.

### Warning

The VGAM family function `logitlaplace1` will not handle a vector of just 0s and 1s as the response; it will only work satisfactorily if the number of trials is large.

See `alaplace1` for other warnings. Care is needed with `tau` values which are too small, e.g., for count data the sample proportion of zeros must be less than all values in `tau`. Similarly, this also holds with `logitlaplace1`, which also requires all `tau` values to be less than the sample proportion of ones.

### Note

The form of input for `logitlaplace1` as response is a vector of proportions (values in [0,1]) and the number of trials is entered into the `weights` argument of `vglm`/`vgam`. See Example 2 below. See `alaplace1` for other notes in general.

### Author(s)

Thomas W. Yee

### References

Kotz, S., Kozubowski, T. J. and Podgorski, K. (2001) The Laplace distribution and generalizations: a revisit with applications to communications, economics, engineering, and finance, Boston: Birkhauser.

Kozubowski, T. J. and Podgorski, K. (2003) Log-Laplace distributions. International Mathematical Journal, 3, 467–495.

Yee, T. W. (2012) Quantile regression for counts and proportions. In preparation.

### See Also

`alaplace1`, `dloglap`.

### Examples

    # Example 1: quantile regression of counts with regression splines
    set.seed(123); my.k <- exp(0)
    adata <- data.frame(x2 = sort(runif(n <- 500)))
    mymu <- function(x) exp( 1 + 3*sin(2*x) / (x+0.5)^2)
    adata <- transform(adata, y = rnbinom(n, mu = mymu(x2), size = my.k))
    mytau <- c(0.1, 0.25, 0.5, 0.75, 0.9); mydof = 3
    # halfstepping is usual:
    fitp <- vglm(y ~ sm.bs(x2, df = mydof), data = adata, trace = TRUE,
                loglaplace1(tau = mytau, parallel.locat = TRUE))

    ## Not run:
    par(las = 1)  # Plot on a log1p() scale
    mylwd <- 1.5

    plot(jitter(log1p(y), factor = 1.5) ~ x2, adata, col = "red", pch = "o",
         main = "Example 1; darkgreen=truth, blue=estimated", cex = 0.75)
    with(adata, matlines(x2, log1p(fitted(fitp)), col = "blue",
                          lty = 1, lwd = mylwd))
    finexgrid <- seq(0, 1, len = 201)
    for (ii in 1:length(mytau))
      lines(finexgrid, col = "darkgreen", lwd = mylwd,
            log1p(qnbinom(p = mytau[ii], mu = mymu(finexgrid), si = my.k)))

    ## End(Not run)
    fitp@extra  # Contains useful information

    # Example 2: sample proportions
    set.seed(123); nnn <- 1000; ssize <- 100  # ssize = 1 will not work!
    adata <- data.frame(x2 = sort(runif(nnn)))
    mymu <- function(x) logitlink( 1.0 + 4*x, inv = TRUE)
    adata <- transform(adata, ssize = ssize,
                        y2 = rbinom(nnn, size = ssize, prob = mymu(x2)) / ssize)

    mytau <- c(0.25, 0.50, 0.75)
    fit1 <- vglm(y2 ~ sm.bs(x2, df = 3), data = adata,
                 weights = ssize, trace = TRUE,
                 logitlaplace1(tau = mytau, lloc = "clogloglink", paral = TRUE))

    ## Not run:
    # Check the solution.  Note: this may be like comparing apples with oranges.
    plotvgam(fit1, se = TRUE, scol = "red", lcol = "blue",
             main = "Truth = 'darkgreen'")
    # Centered approximately !
    linkFunctionChar <- as.character(fit1@misc$link)
    adata <- transform(adata, trueFunction=
                       theta2eta(theta = mymu(x2), link=linkFunctionChar))
    with(adata, lines(x2, trueFunction - mean(trueFunction), col = "darkgreen"))

    # Plot the data + fitted quantiles (on the original scale)
    myylim <- with(adata, range(y2))
    plot(y2 ~ x2, adata, col = "blue", ylim = myylim, las = 1,
         pch = ".", cex = 2.5)
    with(adata, matplot(x2, fitted(fit1), add = TRUE, lwd = 3, type = "l"))
    truecol <- rep(1:3, len = fit1@misc$M)  # Add the 'truth'
    smallxgrid <- seq(0, 1, len = 501)
    for (ii in 1:length(mytau))
      lines(smallxgrid, col = truecol[ii], lwd = 2,
            qbinom(p = mytau[ii], prob = mymu(smallxgrid), size = ssize) / ssize)

    # Plot on the eta (== logitlink()/probit()/...) scale
    with(adata, matplot(x2, predict(fit1), add = FALSE, lwd = 3, type = "l"))
    # Add the 'truth'
    for (ii in 1:length(mytau)) {
      true.quant <- qbinom(p = mytau[ii], pr = mymu(smallxgrid), si = ssize) / ssize
      lines(smallxgrid, theta2eta(theta = true.quant, link = linkFunctionChar),
            col = truecol[ii], lwd = 2)
    }

    ## End(Not run)

[Package VGAM version 1.1-1 Index]