shirleyWilliamsTest {PMCMRplus} | R Documentation

## Shirley-Williams Test

### Description

Performs Shirley's nonparametric equivalent of William's test for contrasting increasing dose levels of a treatment.

### Usage

    shirleyWilliamsTest(x, ...)

    ## Default S3 method:
    shirleyWilliamsTest(
      x,
      g,
      alternative = c("two.sided", "greater", "less"),
      method = c("look-up", "boot"),
      nperm = 10000,
      ...
    )

    ## S3 method for class 'formula'
    shirleyWilliamsTest(
      formula,
      data,
      subset,
      na.action,
      alternative = c("two.sided", "greater", "less"),
      method = c("look-up", "boot"),
      nperm = 10000,
      ...
    )

### Arguments

`x` | a numeric vector of data values, or a list of numeric data vectors.
`...` | further arguments to be passed to or from methods.
`g` | a vector or factor object giving the group for the corresponding elements of `"x"`. Ignored with a warning if `"x"` is a list.
`alternative` | the alternative hypothesis. Defaults to `two.sided`
`method` | a character string specifying the test statistic to use. Defaults to `"look-up"` that uses published Table values of Williams (1972).
`nperm` | number of permutations for the assymptotic permutation test. Defaults to `1000`. Ignored, if `method = "look-up"`.
`formula` | a formula of the form `response ~ group` where `response` gives the data values and `group` a vector or factor of the corresponding groups.
`data` | an optional matrix or data frame (or similar: see `model.frame`) containing the variables in the formula `formula`. By default the variables are taken from `environment(formula)`.
`subset` | an optional vector specifying a subset of observations to be used.
`na.action` | a function which indicates what should happen when the data contain `NA`s. Defaults to `getOption("na.action")`.

### Details

The Shirley-William test is a non-parametric step-down trend test for testing several treatment levels with a zero control. Let there be k groups including the control and let the zero dose level be indicated with i = 0 and the highest dose level with i = m, then the following `m = k - 1` hypotheses are tested:

\begin{array}{ll} \mathrm{H}_{m}: θ_0 = θ_1 = … = θ_m, & \mathrm{A}_{m} = θ_0 ≤ θ_1 ≤ … θ_m, θ_0 < θ_m \\ \mathrm{H}_{m-1}: θ_0 = θ_1 = … = θ_{m-1}, & \mathrm{A}_{m-1} = θ_0 ≤ θ_1 ≤ … θ_{m-1}, θ_0 < θ_{m-1} \\ \vdots & \vdots \\ \mathrm{H}_{1}: θ_0 = θ_1, & \mathrm{A}_{1} = θ_0 < θ_1\\ \end{array}

The procedure starts from the highest dose level (m) to the the lowest dose level (1) and stops at the first non-significant test. The consequent lowest effect dose is the treatment level of the previous test number. This function has included the modifications as recommended by Williams (1986).

If `method = "look-up"` is selected, the function does not return p-values. Instead the critical t-values as given in the tables of Williams (1972) for α = 0.05 (one-sided) are looked up according to the degree of freedoms (v = ∞) and the order number of the dose level (i) and (potentially) modified according to the given extrapolation coefficient β.

Non tabulated values are linearly interpolated with the function `approx`.

For the comparison of the first dose level (i = 1) with the control, the critical z-value from the standard normal distribution is used (`Normal`).

If `method = "boot"`, the p-values are estimated through an assymptotic boot-strap method. The p-values for H_1 are calculated from the t distribution with infinite degree of freedom.

### Value

Either a list with class `"osrt"` or a list with class `"PMCMR"`.

method

  a character string indicating what type of test was performed.

data.name

  a character string giving the name(s) of the data.

statistic

  the estimated statistic(s)

crit.value

  critical R-values for α = 0.05.

alternative

  a character string describing the alternative hypothesis.

parameter

  the parameter(s) of the test distribution.

dist

  a string that denotes the test distribution.

There are print and summary methods available.

A list with class `"PMCMR"` containing the following components:

method

  a character string indicating what type of test was performed.

data.name

  a character string giving the name(s) of the data.

statistic

  lower-triangle matrix of the estimated quantiles of the pairwise test statistics.

p.value

  lower-triangle matrix of the p-values for the pairwise tests.

alternative

  a character string describing the alternative hypothesis.

p.adjust.method

  a character string describing the method for p-value adjustment.

model

  a data frame of the input data.

dist

  a string that denotes the test distribution.

### Note

For `method = "look-up"`, only tests on the level of α = 0.05 can be performed for alternative hypotheses less or greater.

For `method = "boot"` only the alternative `"two.sided"` can be calculated. One may increase the number of permutations to e.g. `nperm = 10000` in order to get more precise p-values. However, this will be on the expense of computational time.

### References

Shirley, E., (1977) Nonparametric Equivalent of Williams Test for Contrasting Increasing Dose Levels of a Treatment, Biometrics 33, 386–389.

Williams, D. A. (1986) Note on Shirley's nonparametric test for comparing several dose levels with a zero-dose control, Biometrics 42, 183–186.

### See Also

`williamsTest`

### Examples

    ## Example from Shirley (1977)
    ## Reaction times of mice to stimuli to their tails.
    y <- c(2.4, 3, 3, 2.2, 2.2, 2.2, 2.2, 2.8, 2, 3,
    2.8, 2.2, 3.8, 9.4, 8.4, 3, 3.2, 4.4, 3.2, 7.4, 9.8, 3.2, 5.8,
    7.8, 2.6, 2.2, 6.2, 9.4, 7.8, 3.4, 7, 9.8, 9.4, 8.8, 8.8, 3.4,
    9, 8.4, 2.4, 7.8)
    g <- gl(4, 10)

    ## Not run:
    ## two.sided test
    summary(shirleyWilliamsTest(y ~ g, method = "boot", alternative = "two.sided"))

    ## End(Not run)

    ## one-sided test using look-up table
    shirleyWilliamsTest(y ~ g, alternative = "greater")

[Package PMCMRplus version 1.6.1 Index]