cao.control {VGAM} | R Documentation

## Control Function for RR-VGAMs (CAO)

### Description

Algorithmic constants and parameters for a constrained additive ordination (CAO), by fitting a reduced-rank vector generalized additive model (RR-VGAM), are set using this function. This is the control function for `cao`.

### Usage

    cao.control(Rank=1, all.knots = FALSE,
                criterion="deviance",
                Cinit=NULL,
                Crow1positive=TRUE,
                epsilon = 1.0e-05,
                Etamat.colmax = 10,
                GradientFunction=FALSE,
                iKvector = 0.1,
                iShape = 0.1,
                Norrr = ~ 1,
                SmallNo = 5.0e-13,
                Use.Init.Poisson.QO=TRUE,
                Bestof = if(length(Cinit)) 1 else 10,
                maxitl = 40,
                method.init = 1,
                bf.epsilon = 1.0e-7,
                bf.maxit = 40,
                Maxit.optim = 250,
                optim.maxit = 20,
                SD.sitescores = 1.0,
                SD.Cinit = 0.02,
                trace = TRUE,
                df1.nl = 2.5, df2.nl = 2.5,
                spar1 = 0, spar2 = 0, ...)

### Arguments

Many of these arguments are identical to `qrrvglm.control`. Here, R is the `Rank`, M is the number of additive predictors, and S is the number of responses (species). Thus M=S for binomial and Poisson responses, and M=2S for the negative binomial and 2-parameter gamma distributions.

`Rank` | The numerical rank R of the model, i.e., the number of latent variables. Currently only `Rank=1` is implemented.
`all.knots` | Logical indicating if all distinct points of the smoothing variables are to be used as knots. Assigning the value `FALSE` means fewer knots are chosen when the number of distinct points is large, meaning less computational expense. See `vgam.control` for details.
`criterion` | Convergence criterion. Currently, only one is supported: the deviance is minimized.
`Cinit` | Optional initial C matrix which may speed up convergence.
`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] and C[1,3] to be negative, and C[1,2] and C[1,4] to be positive.
`epsilon` | Positive numeric. Used to test for convergence for GLMs fitted in FORTRAN. Larger values mean a loosening of the convergence criterion.
`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, the better the initial value. Used only if `Use.Init.Poisson.QO=TRUE`.
`GradientFunction` | Logical. Whether `optim`'s argument `gr` is used or not, i.e., to compute gradient values. Used only if `FastAlgorithm` is `TRUE`. Currently, this argument must be set to `FALSE`.
`iKvector, iShape` | See `qrrvglm.control`.
`Norrr` | Formula giving terms that are not to be included in the reduced-rank regression (or formation of the latent variables). The default is to omit the intercept term from the latent variables. Currently, only `Norrr = ~ 1` is implemented.
`SmallNo` | Positive numeric between `.Machine$double.eps` and `0.0001`. Used to avoid under- or over-flow in the IRLS algorithm.
`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.
`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 works only when the function generates its own initial value for C, i.e., when C are not passed in as initial values. The default is only a convenient minimal number and users are urged to increase this value.
`maxitl` | Positive integer. Maximum number of Newton-Raphson/Fisher-scoring/local-scoring iterations allowed.
`method.init` | See `qrrvglm.control`.
`bf.epsilon` | Positive numeric. Tolerance used by the modified vector backfitting algorithm for testing convergence.
`bf.maxit` | Positive integer. Number of backfitting iterations allowed in the compiled code.
`Maxit.optim` | Positive integer. Number of iterations given to the function `optim` at each of the `optim.maxit` iterations.
`optim.maxit` | Positive integer. Number of times `optim` is invoked.
`SD.sitescores` | Numeric. Standard deviation of the initial values of the site scores, which are generated from a normal distribution. Used when `Use.Init.Poisson.QO` is `FALSE`.
`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`.
`trace` | Logical indicating if output should be produced for each iteration. Having the value `TRUE` is a good idea for large data sets.
`df1.nl, df2.nl` | Numeric and non-negative, recycled to length S. Nonlinear degrees of freedom for smooths of the first and second latent variables. A value of 0 means the smooth is linear. Roughly, a value between 1.0 and 2.0 often has the approximate flexibility of a quadratic. The user should not assign too large a value to this argument, e.g., the value 4.0 is probably too high. The argument `df1.nl` is ignored if `spar1` is assigned a positive value or values. Ditto for `df2.nl`.
`spar1, spar2` | Numeric and non-negative, recycled to length S. Smoothing parameters of the smooths of the first and second latent variables. The larger the value, the more smooth (less wiggly) the fitted curves. These arguments are an alternative to specifying `df1.nl` and `df2.nl`. A value 0 (the default) for `spar1` means that `df1.nl` is used. Ditto for `spar2`. The values are on a scaled version of the latent variables. See Green and Silverman (1994) for more information.
`...` | Ignored at present.

### Details

Allowing the smooths too much flexibility means the CAO optimization problem becomes more difficult to solve. This is because the number of local solutions increases as the nonlinearity of the smooths increases. In situations of high nonlinearity, many initial values should be used, so that `Bestof` should be assigned a larger value. In general, there should be a reasonable value of `df1.nl` somewhere between 0 and about 3 for most data sets.

### Value

A list with the components corresponding to its arguments, after some basic error checking.

### Note

The argument `df1.nl` can be inputted in the format `c(spp1=2, spp2=3, 2.5)`, say, meaning the default value is 2.5, but two species have alternative values.

If `spar1=0` and `df1.nl=0` then this represents fitting linear functions (CLO). Currently, this is handled in the awkward manner of setting `df1.nl` to be a small positive value, so that the smooth is almost linear but not quite. A proper fix to this special case should done in the short future.

### Author(s)

T. W. Yee

### References

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

Green, P. J. and Silverman, B. W. (1994) Nonparametric Regression and Generalized Linear Models: A Roughness Penalty Approach, London: Chapman & Hall.

### See Also

`cao`.

### Examples

    ## Not run:
    hspider[,1:6] = scale(hspider[,1:6]) # Standardized environmental vars
    set.seed(123)
    ap1 = cao(cbind(Pardlugu, Pardmont, Pardnigr, Pardpull, Zoraspin) ~
             WaterCon + BareSand + FallTwig +
             CoveMoss + CoveHerb + ReflLux,
             family = poissonff, data = hspider,
             df1.nl = c(Zoraspin=2.3, 2.1),
             Bestof = 10, Crow1positive = FALSE)
    sort(ap1@misc$deviance.Bestof) # A history of all the iterations

    Coef(ap1)

    par(mfrow=c(2,3)) # All or most of the curves are unimodal; some are
    plot(ap1, lcol="blue") # quite symmetric. Hence a CQO model should be ok

    par(mfrow=c(1,1), las=1)
    index = 1:ncol(ap1@y)  # lvplot is jagged because only 28 sites
    lvplot(ap1, lcol=index, pcol=index, y=TRUE)

    trplot(ap1, label=TRUE, col=index)
    abline(a=0, b=1, lty=2)

    persp(ap1, label=TRUE, col=1:4)
    ## End(Not run)

[Package VGAM version 0.7-9 Index]