shuffle: Unrestricted and restricted permutations

Description Usage Arguments Details Value Author(s) References See Also Examples

Description

Unrestricted and restricted permutation designs for time series, line transects, spatial grids and blocking factors.

Usage

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shuffle(n, control = how())

permute(i, n, control)

Arguments

n

numeric; the length of the returned vector of permuted values. Usually the number of observations under consideration. May also be any object that nobs knows about; see nobs-methods.

control

a list of control values describing properties of the permutation design, as returned by a call to how.

i

integer; row of control$all.perms to return.

Details

shuffle can generate permutations for a wide range of restricted permutation schemes. A small selection of the available combinations of options is provided in the Examples section below.

permute is a higher level utility function for use in a loop within a function implementing a permutation test. The main purpose of permute is to return the correct permutation in each iteration of the loop, either a random permutation from the current design or the next permutation from control$all.perms if it is not NULL and control$complete is TRUE.

Value

For shuffle a vector of length n containing a permutation of the observations 1, ..., n using the permutation scheme described by argument control.

For permute the ith permutation from the set of all permutations, or a random permutation from the design.

Author(s)

Gavin Simpson

References

shuffle() is modelled after the permutation schemes of Canoco 3.1 (ter Braak, 1990); see also Besag & Clifford (1989).

Besag, J. and Clifford, P. (1989) Generalized Monte Carlo significance tests. Biometrika 76; 633–642.

ter Braak, C. J. F. (1990). Update notes: CANOCO version 3.1. Wageningen: Agricultural Mathematics Group. (UR).

See Also

check, a utility function for checking permutation scheme described by how.

Examples

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set.seed(1234)

## unrestricted permutations
shuffle(20)

## observations represent a time series of line transect
CTRL <- how(within = Within(type = "series"))
shuffle(20, control = CTRL)

## observations represent a time series of line transect
## but with mirroring allowed
CTRL <- how(within = Within(type = "series", mirror = TRUE))
shuffle(20, control = CTRL)

## observations represent a spatial grid, 5rx4c
nr <- 5
nc <- 4
CTRL <- how(within = Within(type = "grid", ncol = nc, nrow = nr))
perms <- shuffle(20, control = CTRL)
## view the permutation as a grid
matrix(matrix(1:20, nrow = nr, ncol = nc)[perms],
       ncol = nc, nrow = nr)

## random permutations in presence of strata
plots <- Plots(strata = gl(4, 5))
CTRL <- how(plots = plots, within = Within(type = "free"))
shuffle(20, CTRL)
## as above but same random permutation within strata
CTRL <- how(plots = plots, within = Within(type = "free",
            constant = TRUE))
shuffle(20, CTRL)

## time series within each level of block
CTRL <- how(plots = plots, within = Within(type = "series"))
shuffle(20, CTRL)
## as above, but  with same permutation for each level
CTRL <- how(plots = plots, within = Within(type = "series",
            constant = TRUE))
shuffle(20, CTRL)

## spatial grids within each level of block, 4 x (5r x 5c)
nr <- 5
nc <- 5
nb <- 4 ## number of blocks
plots <- Plots(gl(nb, 25))
CTRL <- how(plots = plots,
            within = Within(type = "grid", ncol = nc, nrow = nr))
shuffle(100, CTRL)
## as above, but with same permutation for each level
CTRL <- how(plots = plots,
            within = Within(type = "grid", ncol = nc, nrow = nr,
                            constant = TRUE))
shuffle(100, CTRL)

## permuting levels of plots instead of observations
CTRL <- how(plots = Plots(gl(4, 5), type = "free"),
            within = Within(type = "none"))
shuffle(20, CTRL)
## permuting levels of plots instead of observations
## but plots represent a time series
CTRL <- how(plots = Plots(gl(4, 5), type = "series"),
            within = Within(type = "none"))
shuffle(20, CTRL)

## permuting levels of plots but plots represent a time series
## free permutation within plots
CTRL <- how(plots = Plots(gl(4, 5), type = "series"),
            within = Within(type = "free"))
shuffle(20, CTRL)

## permuting within blocks
grp <- gl(2, 10) # 2 groups of 10 samples each
CTRL <- how(blocks = grp)
shuffle(length(grp), control = CTRL)

## Simple function using permute() to assess significance
## of a t.test  
pt.test <- function(x, group, control) {
    ## function to calculate t
    t.statistic <- function(x, y) {
        m <- length(x)
        n <- length(y)
        ## means and variances, but for speed
        xbar <- mean(x)
        ybar <- mean(y)
        xvar <- var(x)
        yvar <- var(y)
        pooled <- sqrt(((m-1)*xvar + (n-1)*yvar) / (m+n-2))
        (xbar - ybar) / (pooled * sqrt(1/m + 1/n))
    }
    ## check the control object
    #control <- check(x, control)$control ## FIXME
    ## number of observations
    Nobs <- nobs(x)
    ## group names
    lev <- names(table(group))
    ## vector to hold results, +1 because of observed t
    t.permu <- numeric(length = control$nperm) + 1
    ## calculate observed t
    t.permu[1] <- t.statistic(x[group == lev[1]], x[group == lev[2]])
    ## generate randomisation distribution of t
    for(i in seq_along(t.permu)) {
        ## return a permutation
        want <- permute(i, Nobs, control)
        ## calculate permuted t
        t.permu[i+1] <- t.statistic(x[want][group == lev[1]],
                                    x[want][group == lev[2]])
    }
    ## pval from permutation test
    pval <- sum(abs(t.permu) >= abs(t.permu[1])) / (control$nperm + 1)
    ## return value
    return(list(t.stat = t.permu[1], pval = pval))
}

## generate some data with slightly different means
set.seed(1234)
gr1 <- rnorm(20, mean = 9)
gr2 <- rnorm(20, mean = 10)
dat <- c(gr1, gr2)
## grouping variable
grp <- gl(2, 20, labels = paste("Group", 1:2))
## create the permutation design
control <- how(nperm = 999, within = Within(type = "free"))
## perform permutation t test
perm.val <- pt.test(dat, grp, control)
perm.val

## compare perm.val with the p-value from t.test()
t.test(dat ~ grp, var.equal = TRUE)

permute documentation built on May 29, 2017, 11:21 a.m.