nlme (version 3.1-16)

Variogram.lme: Calculate Semi-variogram for Residuals from an lme Object

Description

This method function calculates the semi-variogram for the within-group residuals from an lme fit. The semi-variogram values are calculated for pairs of residuals within the same group. If collapse is different from "none", the individual semi-variogram values are collapsed using either a robust estimator (robust = TRUE) defined in Cressie (1993), or the average of the values within the same distance interval. The semi-variogram is useful for modeling the error term correlation structure.

Usage

## S3 method for class 'lme':
Variogram(object, distance, form, resType, data, na.action, maxDist,
          length.out, collapse, nint, breaks, robust, metric)

Value

a data frame with columns variog and dist representing, respectively, the semi-variogram values and the corresponding distances. If the semi-variogram values are collapsed, an extra column, n.pairs, with the number of residual pairs used in each semi-variogram calculation, is included in the returned data frame. If object includes a corSpatial element, a data frame with its corresponding semi-variogram is included in the returned value, as an attribute "modelVariog". The returned value inherits from class Variogram.

References

Cressie, N.A.C. (1993), "Statistics for Spatial Data", J. Wiley & Sons.

See Also

lme, Variogram.default, Variogram.gls, plot.Variogram

Examples

data(BodyWeight)
fm1 <- lme(weight ~ Time * Diet, BodyWeight, ~ Time | Rat)
Variogram(fm1, form = ~ Time | Rat, nint = 10, robust = TRUE)