corrFamily: Using corrFamily constructors and descriptors.

In spaMM: Mixed-Effect Models, with or without Spatial Random Effects

Using corrFamily constructors and descriptors.

Description

One can declare and fit correlated random effects belonging to a user-defined correlation (or covariance) model (i.e., a parametric family of correlation matrices, although degenerate case with no parameter are also possible). This documentation is a first introduction to this feature. It is experimental in the sense that its design has been formalized only from a limited number of corrFamily examples, and that the documentation is not mature. Implementing prediction for random-effects defined in this way may be tricky. A distinct documentation corrFamily-design provides more information for the efficient design of new correlation models to be fitted in this way.

A simple example of random-effect model implemented in this way is the autoregressive model of order p (AR(p) in the literature; specifically documented elsewhere, see ARp). It can be used as a formula term like other autocorrelated random-effects predefined in spaMM, to be fitted by fitme or fitmv:

fitme(lh ~ 1 + ARp(1|time, p=3),  # <= declaration of random effect
  < data and other possible arguments >)

User-defined correlation models should be registered for this simple syntax to work (see Details for an alternative syntax):

myARp <- ARp                   # 'myARP' is thus a user-defined model
register_cF("myARp")        # Register it so that the next call works
fitme(lh ~ 1 + myARp(1|time, p=3),  
  < data and other possible arguments >)

The ARp object here copied in myARp is a function (the corrFamily constructor) which returns a list (the corrFamily descriptor) which contains the necessary information to fit a random effect with an AR(p) correlation. The p argument in the myARp(1|time, p=3) term enforces evaluation of myARp(p=3), producing the descriptor for the AR(3) model. The structure of this descriptor is

List of 5
 $ Cf            :function (parvec)  
  ..- < with some attribute >
 $ tpar         : num [1:3] 0.5 0.333 0.25
 $ type         : chr "precision"
 $ initialize   :function (Zmatrix, ...) 
  ..- < with some attribute >
 $ fixed        : NULL
 $ calc_moreargs:function (corrfamily, ...)  
  ..- < with some attribute >
 $ levels_type        : chr "time_series"
 $ calc_corr_from_dist:function (ranFix, char_rd, distmat, ...)  
  ..- < with some attribute >
 < and possibly other elements > 

The meaning of these elements and some additional ones is explained below.

Only Cf and tpar are necessary elements of a corrFamily object. If one designs a new descriptor where some other elements are absent, spaMM will try to provide plausible defaults for these elements. Further, if the descriptor does not provide parameter names (as the names of tpar, or in some more cryptic way), default names "p1", "p2"... will be provided.

Usage

## corrFamily descriptor provided as a list of the form
#
# list(Cf=<.>, tpar=<.>, ...)

## corrFamily constructor: any function that returns 
#    a valid corrFamily descriptor
#
# function(tpar=<.>, fixed=<.>, ...) # typical but not mandatory arguments

## There is a distinct documentation page for 'register_cF'.

Arguments

Elements of the corrFamily descriptor:

Cf	(required): function returning the correlation matrix (or covariance matrix, or their inverse), given its first argument, a parameter vector.
tpar	(required): a feasible argument of Cf. tpar is not an initial nor a fixed value.
type	optional, but required if the return value of Cf is an inverse correlation matrix rather than a correlation matrix, in which case one should specify type="precision".
fixed	optional: fixed values for some correlation parameters, provided as a named vector with names consistent with those to be used for tpar. This is conceived to achieve the same statistical fit as by using the fixed argument of fitme, although the structure of the result of the fit differs in some subtle ways whether parameters are fixed through the descriptor or through the fitting function (see Examples in ARp).
calc_moreargs	optional: a function returning a list with possible elements init, lower and upper for control of estimation (and possibly other elements for other purposes). If the descriptor does not provide this function, a default calc_moreargs will be provided, implementing unbounded optimization.
initialize	optional: a function evaluating variables that may be needed repeatedly by Cf or Af.
Af	This function should be defined if the correlation model requires an A matrix (the middle term in the case the design matrix of a random effect term is described by a triple matrix product ZAL as described in random-effects). Examples can be found in the descriptors returned by the ranGCA and MaternIMRFa constructors.
levels_type	In the above example its value "time_series" informs spaMM that levels of the random effect should be considered for all integer values within the range of the time variable, not only for levels present in the data. If this element is not provided by the constructor, spaMM will internally assume a levels_type suitable for geostatistical models. Further level types may be defined in the future.
calc_corr_from_dist, make_new_corr_lists	Functions possibly needed for prediction (see Details).
need_Cnn	optional: a boolean; default is TRUE. Controls prediction computations (see Details).
public	An environment where some variables can be saved, typically by the initialize expression, for inspection at user level and for re-use. See diallel for an example.

fitting-function arguments:

lower, upper, init and fixed optimization controls can be used to control optimization of continuous parameters as for other random-effect parameters. They are specified as numeric vectors, themselves being element of the corrPars list (see Examples in corrFamily-design). Parameter names (consistent with those to be used for the tpar argument) may be required to disambiguate incomplete vectors (e.g., to specify only its second element). Apart from fixed ones, any of the values not specified through the fitting-function arguments will be sought in the return value of the calc_moreargs function, if provided in the descriptor. If the lower or upper information is missing there, it must be provided throught the fitting-function call. If the init information is missing, a default value will be deduced from the bounds. The init specification is thus always optional while the bounds specification is optional only if the descriptor provides default values.

Arguments of the corrFamily constructor

These may be ad libitum, as design rules are defined only for the returned descriptor. However, arguments tpar, fixed, and public of predefined constructors, such as ARp, are designed to match the above respective elements of the descriptor.

Details

* Constructor elements for prediction:

For prediction of autocorrelated random effects, one must first assess whether levels of the random effect not represented in the fit are possible in new data (corresponding to new spatial locations in geostatistical models, or new time steps in time series). In that case need_Cnn must be TRUE (Interpolated MRFs do not require this as all required random-effect levels are determined by the IMRF mesh argument rather than by the fitted data or new data).

Next, for autocorrelated random effects where need_Cnn is TRUE, a make_new_corr_lists function must be provided, except when a calc_corr_from_dist function is instead provided (which may be sufficient for models that construct the correlation from a spatial distance matrix). When need_Cnn is FALSE, a make_new_corr_lists function may still be needed.

The Examples section provides a simple example of such design, and the source code of the ARp or ARMA constructors provide further examples. They show that the make_new_corr_lists function may assign matrices or vectors as elements of several lists contained in a newLv_env environment. A matrix is assigned in the cov_newLv_oldv_list, specifying correlations between “new” levels of the random effect (implied by the new data) and “old” levels (those already included in the design matrix of the random effect for the fit). If need_Cnn is TRUE, a second matrix may be assigned in the cov_newLv_newLv_list, specifying correlation between “new” levels, and the diagonal of this matrix is assigned in the diag_cov_newLv_newLv_list. The overall structure of the code (the conditions where these assignments are performed, and the list indices), should be conserved.

When calling simulate(., newdata=<non-NULL>, type="marginal", a fourth matrix may be useful, assigned into a L_newLv_newLv_list, specifying the matrix root (as a tcrossprod factor) of the correlation matrix stored in cov_newLv_newLv_list. The relevant spaMM procedure will however try to compute it on the fly when it has not been provided by the make_new_corr_lists function.

* Fitting a corrFamily without a constructor:

It is possible to use an unregistered corrFamily, as follows:

AR3 <- ARp(p=3)          # Generate descriptor of AR model of order 3

fitme(lh ~ 1 + corrFamily(1|time),  # <= declaration of random effect
  covStruct=list( 
    corrFamily= AR3     # <= declaration of its correlation structure
  ), 
  < data and other possible arguments >)

Here the fit only uses a descriptor list, not a constructor function. This descriptor is here provided to the fitting function as an element of the covStruct argument (using the general syntax of this argument), and in the model formula the corresponding random effect is declared as a term of the form
corrFamily(1|<grouping factor>).

This syntax is more complex than the one using a registered constructor, but it might be useful for development purposes (one only has to code the descriptor, not the constructor function). However, it is not general; in particular, using registered constructors may be required to obtain correct results when fitting multivariate-response models by fitmv.

See Also

See ARp, diallel, and MaternIMRFa for basic examples of using a predefined corrFamily descriptor, and corrFamily-design for more geeky stuff including examples of implementing simple new correlation families.

Examples

## Not run: 
### Minimal (with many features missing) reimplementation 
#     of corrMatrix() terms as a corrFamily 


corrMatrix_cF <- function(corrMatrix) {
  
  force(corrMatrix) # Makes it available in the environment of the functions next defined.
  oldZlevels <- NULL
  
  initialize <- function(Zmatrix, ...) {
    oldZlevels <<- colnames(Zmatrix) # Pass info about levels of the random effect in the data.
  }
  
  Cf <- function(newlevels=oldZlevels ) {
    if (length(newlevels)) {
      corrMatrix[newlevels,newlevels]
    } else corrMatrix[oldZlevels,oldZlevels] # for Cf(tpar=numeric(0L))
  }
  
  calc_moreargs <- function(corrfamily, ...) {
    list(init=c(),lower=c(),upper=c())
  }
  
  make_new_corr_lists <- function(newLv_env, which_mats, newZAlist, new_rd, ...) {
    newlevels <- colnames(newZAlist[[new_rd]])
    newLv_env$cov_newLv_oldv_list[[new_rd]] <- corrMatrix[newlevels,oldZlevels, drop=FALSE]
    if (which_mats$nn[new_rd]) {
      newLv_env$cov_newLv_newLv_list[[new_rd]] <- corrMatrix[newlevels,newlevels, drop=FALSE]
    } else { 
      newLv_env$diag_cov_newLv_newLv_list[[new_rd]] <- rep(1,length(newlevels)) 
    }
  }
  
  list(Cf=Cf, tpar=numeric(0L), initialize=initialize, calc_moreargs=calc_moreargs, 
       make_new_corr_lists=make_new_corr_lists,
       tag="corrMatrix_cF") 
}

register_cF("corrMatrix_cF")

# usage:

data("blackcap") 
MLcorMat <- MaternCorr(proxy::dist(blackcap[,c("latitude","longitude")]),
                       nu=0.6285603,rho=0.0544659)
corrmat <- proxy::as.matrix(MLcorMat, diag=1)

fitme(migStatus ~ means+ corrMatrix_cF(1|name, corrMatrix=corrmat),data=blackcap,
      corrMatrix=MLcorMat,method="ML")
      
unregister_cF("corrMatrix_cF") # Tidy things before leaving.         


## End(Not run)

spaMM documentation built on June 22, 2024, 9:48 a.m.