rq.fit.scad {quantreg} | R Documentation

## SCADPenalized Quantile Regression

### Description

The fitting method implements the smoothly clipped absolute deviation penalty of Fan and Li for fitting quantile regression models. When the argument `lambda` is a scalar the penalty function is the scad modified l1 norm of the last (p-1) coefficients, under the presumption that the first coefficient is an intercept parameter that should not be subject to the penalty. When `lambda` is a vector it should have length equal the column dimension of the matrix `x` and then defines a coordinatewise specific vector of scad penalty parameters. In this case `lambda` entries of zero indicate covariates that are not penalized. There should be a sparse version of this, but isn't (yet).

### Usage

    rq.fit.scad(x, y, tau = 0.5, alpha = 3.2, lambda = 1, start="rq",
    	beta = .9995, eps = 1e-06)

### Arguments

`x` | the design matrix
`y` | the response variable
`tau` | the quantile desired, defaults to 0.5.
`alpha` | tuning parameter of the scad penalty.
`lambda` | the value of the penalty parameter that determines how much shrinkage is done. This should be either a scalar, or a vector of length equal to the column dimension of the `x` matrix.
`start` | starting method, default method 'rq' uses the unconstrained rq estimate, while method 'lasso' uses the corresponding lasso estimate with the specified lambda.
`beta` | step length parameter for Frisch-Newton method.
`eps` | tolerance parameter for convergence.

### Details

The algorithm is an adaptation of the "difference convex algorithm" described in Wu and Liu (2008). It solves a sequence of (convex) QR problems to approximate solutions of the (non-convex) scad problem.

### Value

Returns a list with a coefficient, residual, tau and lambda components. When called from `"rq"` as intended the returned object has class "scadrqs".

### Author(s)

R. Koenker

### References

Wu, Y. and Y. Liu (2008) Variable Selection in Quantile Regression, Statistica Sinica, to appear.

### See Also

`rq`

### Examples

    n <- 60
    p <- 7
    rho <- .5
    beta <- c(3,1.5,0,2,0,0,0)
    R <- matrix(0,p,p)
    for(i in 1:p){
            for(j in 1:p){
                    R[i,j] <- rho^abs(i-j)
                    }
            }
    set.seed(1234)
    x <- matrix(rnorm(n*p),n,p) %*% t(chol(R))
    y <- x %*% beta + rnorm(n)

    f <- rq(y ~ x, method="scad",lambda = 30)
    g <- rq(y ~ x, method="scad", start = "lasso", lambda = 30)

[Package quantreg version 5.21 Index]