CovMest: Constrained M-Estimates of Location and Scatter

Description

Computes constrained M-Estimates of multivariate location and scatter based on the translated biweight function (‘t-biweight’) using a High breakdown point initial estimate as defined by Rocke (1996). The default initial estimate is the Minimum Volume Ellipsoid computed with CovMve. The raw (not reweighted) estimates are taken and the covariance matrix is standardized to determinant 1.

Usage

CovMest(x, r = 0.45, arp = 0.05, eps=1e-3,
        maxiter=120, control, t0, S0, initcontrol)

Arguments

a matrix or data frame.

required breakdown point. Allowed values are between (n - p)/(2 * n) and 1 and the default is 0.45

arp

asympthotic rejection point, i.e. the fraction of points receiving zero weight (see Rocke (1996)). Default is 0.05.

eps

a numeric value specifying the relative precision of the solution of the M-estimate. Defaults to 1e-3

maxiter

maximum number of iterations allowed in the computation of the M-estimate. Defaults to 120

control

a control object (S4) of class CovControlMest-class containing estimation options - same as these provided in the fucntion specification. If the control object is supplied, the parameters from it will be used. If parameters are passed also in the invocation statement, they will override the corresponding elements of the control object.

optional initial high breakdown point estimates of the location. If not supplied MVE will be used.

optional initial high breakdown point estimates of the scatter. If not supplied MVE will be used.

initcontrol

optional control object - of class CovControl - specifing the initial high breakdown point estimates of location and scatter. If not supplied MVE will be used.

Value

An object of class CovMest-class which is a subclass of the virtual class CovRobust-class.

Details

Rocke (1996) has shown that the S-estimates of multivariate location and scatter in high dimensions can be sensitive to outliers even if the breakdown point is set to be near 0.5. To mitigate this problem he proposed to utilize the translated biweight (or t-biweight) method with a standardization step consisting of equating the median of rho(d) with the median under normality. This is then not an S-estimate, but is instead a constrained M-estimate. In order to make the smooth estimators to work, a reasonable starting point is necessary, which will lead reliably to a good solution of the estimator. In CovMest the MVE computed by CovMve is used, but the user has the possibility to give her own initial estimates.

References

D.L.Woodruff and D.M.Rocke (1994) Computable robust estimation of multivariate location and shape on high dimension using compound estimators, Journal of the American Statistical Association, 89, 888--896.

D.M.Rocke (1996) Robustness properties of S-estimates of multivariate location and shape in high dimension, Annals of Statistics, 24, 1327-1345.

D.M.Rocke and D.L.Woodruff (1996) Identification of outliers in multivariate data Journal of the American Statistical Association, 91, 1047--1061.

Todorov V & Filzmoser P (2009), An Object Oriented Framework for Robust Multivariate Analysis. Journal of Statistical Software, 32(3), 1--47. URL http://www.jstatsoft.org/v32/i03/.

Examples

Run this code

# NOT RUN {
library(rrcov)
data(hbk)
hbk.x <- data.matrix(hbk[, 1:3])
CovMest(hbk.x)

## the following four statements are equivalent
c0 <- CovMest(hbk.x)
c1 <- CovMest(hbk.x, r = 0.45)
c2 <- CovMest(hbk.x, control = CovControlMest(r = 0.45))
c3 <- CovMest(hbk.x, control = new("CovControlMest", r = 0.45))

## direct specification overrides control one:
c4 <- CovMest(hbk.x, r = 0.40,
             control = CovControlMest(r = 0.25))
c1
summary(c1)
plot(c1)
# }

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