VGAM (version 0.9-0)

qrrvglm.control: Control function for QRR-VGLMs (CQO)

Description

Algorithmic constants and parameters for a constrained quadratic ordination (CQO), by fitting a quadratic reduced-rank vector generalized linear model (QRR-VGLM), are set using this function. It is the control function for cqo.

Usage

qrrvglm.control(Rank = 1,
                Bestof = if(length(Cinit)) 1 else 10,
                checkwz = TRUE,
                Cinit = NULL,
                Crow1positive = TRUE,
                epsilon = 1.0e-06,
                EqualTolerances = TRUE,
                Etamat.colmax = 10,
                FastAlgorithm = TRUE,
                GradientFunction = TRUE,
                Hstep = 0.001,
                isdlv = rep(c(2, 1, rep(0.5, length = Rank)), length = Rank),
                iKvector = 0.1,
                iShape = 0.1,
                ITolerances = FALSE,
                maxitl = 40,
                imethod = 1,
                Maxit.optim = 250,
                MUXfactor = rep(7, length=Rank),
                Norrr = ~ 1,
                optim.maxit = 20,
                Parscale = if(ITolerances) 0.001 else 1.0,
                SD.Cinit = 0.02,
                SmallNo = 5.0e-13, 
                trace = TRUE,
                Use.Init.Poisson.QO = TRUE, 
                wzepsilon = .Machine$double.eps^0.75, ...)

Arguments

Rank

The numerical rank $R$ of the model, i.e., the number of ordination axes. Must be an element from the set {1,2,...,min($M$,$p_2$)} where the vector of explanatory variables $x$ is partitioned into ($x_1$,$x_2$), which is of dimension $p_1+

Bestof

Integer. The best of Bestof models fitted is returned. This argument helps guard against local solutions by (hopefully) finding the global solution from many fits. The argument has value 1 if an initial value for $C$ is inputted u

checkwz

logical indicating whether the diagonal elements of the working weight matrices should be checked whether they are sufficiently positive, i.e., greater than wzepsilon. If not, any values less than wzepsilon are replac

Cinit

Optional initial $C$ matrix, which must be a $p_2$ by $R$ matrix. The default is to apply .Init.Poisson.QO() to obtain initial values.

Crow1positive

Logical vector of length Rank (recycled if necessary): are the elements of the first row of $C$ positive? For example, if Rank is 4, then specifying Crow1positive=c(FALSE, TRUE) will force $C[1,1]$

epsilon

Positive numeric. Used to test for convergence for GLMs fitted in C. Larger values mean a loosening of the convergence criterion. If an error code of 3 is reported, try increasing this value.

EqualTolerances

Logical indicating whether each (quadratic) predictor will have equal tolerances. Having EqualTolerances = TRUE can help avoid numerical problems, especially with binary data. Note that the estimated (common) tolerance matri

Etamat.colmax

Positive integer, no smaller than Rank. Controls the amount of memory used by .Init.Poisson.QO(). It is the maximum number of columns allowed for the pseudo-response and its weights. In general, the larger the value,

FastAlgorithm

Logical. Whether a new fast algorithm is to be used. The fast algorithm results in a large speed increases compared to Yee (2004). Some details of the fast algorithm are found in Appendix A of Yee (2006). Setting FastAlgorithm = FALSE

GradientFunction

Logical. Whether optim's argument gr is used or not, i.e., to compute gradient values. Used only if FastAlgorithm is TRUE. The default value is usually fas

Hstep

Positive value. Used as the step size in the finite difference approximation to the derivatives by optim.

isdlv

Initial standard deviations for the latent variables (site scores). Numeric, positive and of length $R$ (recycled if necessary). This argument is used only if ITolerances = TRUE. Used by .Init.Poisson.QO() to obtain init

iKvector, iShape

Numeric, recycled to length $S$ if necessary. Initial values used for estimating the positive $k$ and $\lambda$ parameters of the negative binomial and 2-parameter gamma distributions respectively. For further information see

ITolerances

Logical. If TRUE then the (common) tolerance matrix is the $R$ by $R$ identity matrix by definition. Note that having ITolerances = TRUE implies EqualTolerances = TRUE, but not vice versa. Internally, the qu

maxitl

Maximum number of times the optimizer is called or restarted. Most users should ignore this argument.

imethod

Method of initialization. A positive integer 1 or 2 or 3 etc. depending on the VGAM family function. Currently it is used for negbinomial and

Maxit.optim

Positive integer. Number of iterations given to the function optim at each of the optim.maxit iterations.

MUXfactor

Multiplication factor for detecting large offset values. Numeric, positive and of length $R$ (recycled if necessary). This argument is used only if ITolerances = TRUE. Offsets are $-0.5$ multiplied by the sum of the squares of all $R

optim.maxit

Positive integer. Number of times optim is invoked. At iteration i, the ith value of Maxit.optim is fed into optim<

Norrr

Formula giving terms that are not to be included in the reduced-rank regression (or formation of the latent variables), i.e., those belong to $x_1$. Those variables which do not make up the latent variable (reduced-rank regression

Parscale

Numerical and positive-valued vector of length $C$ (recycled if necessary). Passed into optim(..., control=list(parscale=Parscale)); the elements of $C$ become $C$ / Parscale. Setting ITolerances = TRUE r

SD.Cinit

Standard deviation of the initial values for the elements of $C$. These are normally distributed with mean zero. This argument is used only if Use.Init.Poisson.QO = FALSE and $C$ is not inputted using Cinit

trace

Logical indicating if output should be produced for each iteration. The default is TRUE because the calculations are numerically intensive, meaning it may take a long time, so that the user might think the computer has locked

SmallNo

Positive numeric between .Machine$double.eps and 0.0001. Used to avoid under- or over-flow in the IRLS algorithm. Used only if FastAlgorithm is TRUE.

Use.Init.Poisson.QO

Logical. If TRUE then the function .Init.Poisson.QO() is used to obtain initial values for the canonical coefficients $C$. If FALSE then random numbers are used instead.

wzepsilon

Small positive number used to test whether the diagonals of the working weight matrices are sufficiently positive.

...

Ignored at present.

Value

• A list with components matching the input names.

Warning

The default value of Bestof is a bare minimum for many datasets, therefore it will be necessary to increase its value to increase the chances of obtaining the global solution.

Details

Recall that the central formula for CQO is $$\eta = B_1^T x_1 + A \nu + \sum_{m=1}^M (\nu^T D_m \nu) e_m$$ where $x_1$ is a vector (usually just a 1 for an intercept), $x_2$ is a vector of environmental variables, $\nu=C^T x_2$ is a $R$-vector of latent variables, $e_m$ is a vector of 0s but with a 1 in the $m$th position. QRR-VGLMs are an extension of RR-VGLMs and allow for maximum likelihood solutions to constrained quadratic ordination (CQO) models.

Having ITolerances = TRUE means all the tolerance matrices are the order-$R$ identity matrix, i.e., it forces bell-shaped curves/surfaces on all species. This results in a more difficult optimization problem (especially for 2-parameter models such as the negative binomial and gamma) because of overflow errors and it appears there are more local solutions. To help avoid the overflow errors, scaling $C$ by the factor Parscale can help enormously. Even better, scaling $C$ by specifying isdlv is more understandable to humans. If failure to converge occurs, try adjusting Parscale, or better, setting EqualTolerances = TRUE (and hope that the estimated tolerance matrix is positive-definite). To fit an equal-tolerances model, it is firstly best to try setting ITolerances = TRUE and varying isdlv and/or MUXfactor if it fails to converge. If it still fails to converge after many attempts, try setting EqualTolerances = TRUE, however this will usually be a lot slower because it requires a lot more memory.

With a $R > 1$ model, the latent variables are always uncorrelated, i.e., the variance-covariance matrix of the site scores is a diagonal matrix.

If setting EqualTolerances = TRUE is used and the common estimated tolerance matrix is positive-definite then that model is effectively the same as the ITolerances = TRUE model (the two are transformations of each other). In general, ITolerances = TRUE is numerically more unstable and presents a more difficult problem to optimize; the arguments isdlv and/or MUXfactor often must be assigned some good value(s) (possibly found by trial and error) in order for convergence to occur. Setting ITolerances = TRUE forces a bell-shaped curve or surface onto all the species data, therefore this option should be used with deliberation. If unsuitable, the resulting fit may be very misleading. Usually it is a good idea for the user to set EqualTolerances = FALSE to see which species appear to have a bell-shaped curve or surface. Improvements to the fit can often be achieved using transformations, e.g., nitrogen concentration to log nitrogen concentration.

Fitting a CAO model (see cao) first is a good idea for pre-examining the data and checking whether it is appropriate to fit a CQO model.

References

Yee, T. W. (2004) A new technique for maximum-likelihood canonical Gaussian ordination. Ecological Monographs, 74, 685--701.

Yee, T. W. (2006) Constrained additive ordination. Ecology, 87, 203--213.

See Also

cqo, rcqo, Coef.qrrvglm, Coef.qrrvglm-class, optim, binomialff, poissonff, negbinomial, gamma2, gaussianff.

Examples

# Poisson CQO with equal tolerances
set.seed(111) # This leads to the global solution
hspider[,1:6] <- scale(hspider[,1:6]) # Good idea when ITolerances = TRUE
p1 <- cqo(cbind(Alopacce, Alopcune, Alopfabr, Arctlute, Arctperi, Auloalbi,
               Pardlugu, Pardmont, Pardnigr, Pardpull, Trocterr, Zoraspin) ~
          WaterCon + BareSand + FallTwig + CoveMoss + CoveHerb + ReflLux,
          quasipoissonff, data = hspider, EqualTolerances = TRUE)
sort(p1@misc$deviance.Bestof) # A history of all the iterations

(isdlv <- apply(lv(p1), 2, sd)) # Should be approx isdlv
 
# Refit the model with better initial values
set.seed(111)  # This leads to the global solution
p1 <- cqo(cbind(Alopacce, Alopcune, Alopfabr, Arctlute, Arctperi, Auloalbi, 
               Pardlugu, Pardmont, Pardnigr, Pardpull, Trocterr, Zoraspin) ~
          WaterCon + BareSand + FallTwig + CoveMoss + CoveHerb + ReflLux,
          ITolerances = TRUE, isdlv = isdlv, # Note the use of isdlv here
          quasipoissonff, data = hspider)
sort(p1@misc$deviance.Bestof) # A history of all the iterations