aovp: Fitting and testing ANOVA using permutation tests

In lmPerm: Permutation Tests for Linear Models

Description

aovp is aov modified to use permutation tests instead of normal theory tests. Like aov, the ANOVA model is fitted by a call to lmp for each stratum. Timing differences between aovp and aov are negligible.

Usage

aovp(formula, data = NULL, perm="Exact", seqs=FALSE, 
              center=TRUE, projections = FALSE, qr = TRUE,
              contrasts = NULL, ...)

Arguments

The arguments are the same as for aov. Additional parameters may be included. They are described in the "Additional Parameters" section below. These additional parameters are the same as for lmp.

formula	A formula specifying the model.
data	A data frame in which the variables specified in the formula will be found. If missing, the variables are searched for in the standard way.
perm	"Exact", "Prob", "SPR" will produce permutation probabilities. Anything else, such as "", will produce F-test probabilities.
seqs	If TRUE, will calculate sequential SS. If FALSE, unique SS.
center	If TRUE, numerical variables will be centered.
projections	Logical flag: should the projections be returned?
qr	Logical flag: should the QR decomposition be returned?
contrasts	A list of contrasts to be used for some of the factors in the formula. These are not used for any Error term, and supplying contrasts for factors only in the Error term will give a warning.
...	Arguments to be passed to aovp, such as those listed below.

Details

The model Y=Xb+Zg+e is assumed, where X is the incidence matrix for fixed effects, and Z is an incidence matrix for random effects, with columns representing the several error strata. The aovp() algorithm projects Y into strata such that each stratum has a single error term, such as a stratum defined by whole blocks. X is also projected so that the model in this stratum becomes P(Y)=P(X)bi+ei.

The vector bi is divided into sources with dfj degrees of freedom for the jth source, and summary(aovp()) will produce an ANOVA table for these sources in the ith strata. See Venables and Ripley for details.

Either permutation test p-values or the usual F-test p-values will be output. Polynomial model terms are collected into sources, so that Y~A+B+I(A^2) will contain two sources, one for A with 2 df, and one for B with 1 df. Sources for factors are treated as usual, and polynomial terms and factors may be mixed in one model. The function poly.formula may be used to create polynomial models, and the function multResp may be used to create multiresponse matrices for the lhs from variables defined in data.

The Exact method will permute the values exactly. The Prob and SPR methods will approximate the permutation distribution by randomly exchanging pairs of Y elements. The Exact method will be used by default when the number of observations is less than or equal to maxExact, otherwise Prob will be used.

Prob: Iterations terminate when the estimated standard error of the estimated proportion p is less than p*Ca. The iteration continues until all sources and coefficients meet this criterion or until maxIter is reached. See Anscome(1953) for the origin of the criterion.

SPR: This method uses sequential probability ratio tests to decide between the hypotheses p0 and p1 for a strength (alpha, beta) test. The test terminates upon the acceptance or rejection of p0 or if maxIter is reached. See Wald (1947). The power of the SPR is beta at p0 and increases to 1-beta at p1. Placing p0 and p1 close together makes the cut off sharp.

Exact: This method generates all permutations of Y. It will generally be found too time consuming for more than 10 or 11 observations, but note that aovp may be used to divide the data into small enough blocks for which exact permutation tests may be possible.

For Prob and SPR, one may set nCycle to unity to exchange all elements instead of just pairs at each iteration, but there seems to be no advantage to doing this unless the number of iterations is small – say less than 100.

The SS will be calculated sequentially, just as lm does; or they may be calculated uniquely, which means that the SS for each source is calculated conditionally on all other sources. This is SAS type III, which is also what drop1() produces, except that drop1() will not drop main effects when interactions are present. The parameter seqs may be used to override the default unique calculation behavior.

Value

The usual output from aov, with permutation p-values instead of normal theory p-values.

Iter	For Prob and SPR: The number of iterations until the criterion is met.
Accept	For SPR: 1 the p0 hypothesis is accepted, and 0 for rejection or when no decision before maxIter occurs.

Additional Parameters

These are the same as for lmp.

settings

If TRUE, settings such as sequential or unique will be printed. Default TRUE

useF

If TRUE, SS/Resid SS will be used, otherwise SS. The default is TRUE

maxIter

For Prob and SPR: The maximum number of iterations. Default 1000.

Ca

For Prob: Stop iterations when estimated standard error of the estimated p is less than Ca*p. Default 0.1

p0

For SPR: Null hypothesis probability. Default 0.05

p1

For SPR: Alternative hypothesis probability. Default 0.06

alpha

For SPR: Size of SPR test. Default 0.01

beta

For SPR: Type II error for SPR test. Default 0.01

maxExact

For Exact: maximum number of observations allowed. If data exceeds this, Prob is used. Default 10.

nCycle

For Prob and SPR: Performs a complete random permutation, instead of pairwise exchanges, every nCycle cycles. Default 1000.

Note

There is a vignette with more details and an example. To access it, type

vignette("lmPerm")

The default contrasts are set internally to (contr.sum, contr.poly), which means that coefficients are either pairwise contrasts with the last level or polynomial contrasts.

Numerical variables should be centered in order to make them orthogonal to the constant when ANOVA is to be done.

Factors with many levels may cause problems for the methodology used by R. It is designed to work well with both balanced and unbalanced data, but it is best used for factors with no more than four or five levels. If difficulties occur, degrees of freedom for high order sources will be reduced, and sometimes the sources will be omitted entirely. An error message usually accompanies such behavior.

The variables inside the Error() term are treated in the order given. Thus Error(A+B) will usually produce components in the order A,B, with B orthogonalized with respect to A. This may cause confusion for unbalanced block structures.

This function will behave identically to aov() if the following parameters are set: perm="", seq=TRUE, center=FALSE.

Author(s)

Bob Wheeler [e-mail]

References

Hald, A. 1952

Statistical theory with engineering applications. Wiley. Table 7.4

Venables, W.N, and Ripley, B. D. (2000)

Modern applied statistics with S-Plus, p176

Wald, A. (1947)

Sequential analysis, Wiley, Sec. 5.3

See Also

summary.aovp, lmp

Examples

## A simple randomized block example. 
# There are 7 blocks and 6 treatments. A first
# analysis with blocks as a factor shows block to be significant and treatments not. 

data(Hald17.4)
summary(aovp(Y~T+block,Hald17.4))

# Using the block to define a separate error strata tells a different story.

summary(aovp(Y~T+Error(block),Hald17.4))

# There appears to be a linear trend in the blocks. This may be investigated by
# extracting a linear component. The factor L was created by copying the block
# factor and assigning it a linear contrast, like this
# contrasts(L,1)<-contr.poly(7). The analysis then becomes.

summary(aovp(Y~T+L+Error(block),Hald17.4))

# The L factor is not significant under permutation. It is significant when aov()
# is used and the test is the usual F-test.


## From Venables and Ripley (2000)
# This is a 2^3 factorial in the variables N,P,K. It is fractioned by using the 
# three way interaction, NPK, into two fractions of 4. Each of these fractions is 
# allocated to 3 blocks, making 6 blocks in all. An analysis with block as a 
# variable is the following. As may be seen, aovp() discards the confounded NPK interaction.

data(NPK)
summary(aovp(yield ~ block + N*P*K, NPK))

# Since the NPK interaction was confounded with blocks, the experimenter no doubt judged 
# it of lesser interest. It may however be examined by including blocks as an additional
# error term as follows. The basic error level between blocks is of course larger than 
# that within blocks, so the NPK interaction would have to be substantially larger that
# it would have had to be were it tested within blocks.
summary(aovp(yield ~  N*P*K + Error(block), NPK))

# The SS calculated by aovp() are unique SS by default. That is,
# they are sums of squares for the difference of a model with and without the source. The
# resulting test is a test of the hypothesis that the source has no effect on the response.
# Sequential SS, which are those produced by aov() may be obtained by setting the
# parameter seqs=TRUE. simDesign is an unbalanced design created by the AlgDesign package.

data(simDesign)
summary(aovp(Y~.,simDesign))
summary(aovp(Y~.,simDesign,seqs=TRUE)) 

# Since there is only one stratum, these results are the same as would be obtained from
anova(lmp(Y~.,simDesign))

# ANOVA for numerical variables. First using contrasts, then numeric variables.
data(Federer276)
summary(aovp(Plants~Variety*Treatment+Error(Rep/Plot),Federer276))
data(Federer276Numeric)
summary(aovp(poly.formula(Plants~quad(Variety,Treatment)+Error(Rep/Plot)),Federer276Numeric))
# The coefficients and their p-values may be obtained by
summaryC(aovp(Plants~Variety*Treatment+Error(Rep/Plot),Federer276))

lmPerm documentation built on May 2, 2019, 12:35 p.m.