chemometrics (version 1.4.1)

mvr_dcv: Repeated double-cross-validation for PLS and PCR

Description

Performs a careful evaluation by repeated double-CV for multivariate regression methods, like PLS and PCR.

Usage

mvr_dcv(formula, ncomp, data, subset, na.action,  method = c("kernelpls", "widekernelpls", "simpls", "oscorespls", "svdpc"),  scale = FALSE, repl = 100, sdfact = 2,  segments0 = 4, segment0.type = c("random", "consecutive", "interleaved"),  length.seg0, segments = 10, segment.type = c("random", "consecutive", "interleaved"),  length.seg, trace = FALSE, plot.opt = FALSE, selstrat = "hastie", ...)

Arguments

formula

formula, like y~X, i.e., dependent~response variables

ncomp

number of PLS components

data

data frame to be analyzed

subset

optional vector to define a subset

na.action

a function which indicates what should happen when the data contain missing values

method

the multivariate regression method to be used, see mvr

scale

numeric vector, or logical. If numeric vector, X is scaled by dividing each variable with the corresponding element of 'scale'. If 'scale' is 'TRUE', X is scaled by dividing each variable by its sample standard deviation. If cross-validation is selected, scaling by the standard deviation is done for every segment.

repl

Number of replicattion for the double-CV

sdfact

factor for the multiplication of the standard deviation for the determination of the optimal number of components

segments0

the number of segments to use for splitting into training and test data, or a list with segments (see mvrCv)

segment0.type

the type of segments to use. Ignored if 'segments0' is a list

length.seg0

Positive integer. The length of the segments to use. If specified, it overrides 'segments' unless 'segments0' is a list

segments

the number of segments to use for selecting the optimal number if components, or a list with segments (see mvrCv)

segment.type

the type of segments to use. Ignored if 'segments' is a list

length.seg

Positive integer. The length of the segments to use. If specified, it overrides 'segments' unless 'segments' is a list

trace

logical; if 'TRUE', the segment number is printed for each segment

plot.opt

if TRUE a plot will be generated that shows the selection of the optimal number of components for each step of the CV

selstrat

method that defines how the optimal number of components is selected, should be one of "diffnext", "hastie", "relchange"; see details

...

additional parameters

Value

Details

In this cross-validation (CV) scheme, the optimal number of components is determined by an additional CV in the training set, and applied to the test set. The procedure is repeated repl times. There are different strategies for determining the optimal number of components (parameter selstrat): "diffnext" compares MSE+sdfact*sd(MSE) among the neighbors, and if the MSE falls outside this bound, this is the optimal number. "hastie" searches for the number of components with the minimum of the mean MSE's. The optimal number of components is the model with the smallest number of components which is still in the range of the MSE+sdfact*sd(MSE), where MSE and sd are taken from the minimum. "relchange" is a strategy where the relative change is combined with "hastie": First the minimum of the mean MSE's is searched, and MSE's of larger components are omitted. For this selection, the relative change in MSE compared to the min, and relative to the max, is computed. If this change is very small (e.g. smaller than 0.005), these components are omitted. Then the "hastie" strategy is applied for the remaining MSE's.

References

K. Varmuza and P. Filzmoser: Introduction to Multivariate Statistical Analysis in Chemometrics. CRC Press, Boca Raton, FL, 2009.

See Also

mvr

Examples

data(NIR)
X <- NIR$xNIR[1:30,]      # first 30 observations - for illustration
y <- NIR$yGlcEtOH[1:30,1] # only variable Glucose
NIR.Glc <- data.frame(X=X, y=y)
res <- mvr_dcv(y~.,data=NIR.Glc,ncomp=10,method="simpls",repl=10)