twilight.pval {twilight}R Documentation

A function to compute p-values from expression sets

Description

A function to compute two-sample t, Z and fold change equivalent test statistics (paired or unpaired) and correlation coefficients. Based on permutations, expected test statistics as given in Tusher et al. (2001) and empirical p-values are computed. Additional output are q-values computed as given in Storey and Tibshirani (2003). The resulting object is of class twilight and can be passed to functions twilight or plot.twilight.

Usage

twilight.pval(xin, yin, method = "fc", paired = FALSE, B = 1000, yperm = NULL, balance = FALSE, quant.ci = 0.95, s0=NULL, verbose = TRUE, filtering = FALSE)

Arguments

xin Either an expression set (exprSet) or a data matrix with rows corresponding to features and columns corresponding to samples.
yin A numerical vector containing class labels. The higher label denotes the case, the lower label the control samples to test case vs. control. For correlation scores, yin can be any numerical vector of length equal to the number of samples.
method Character string: "fc" for fold change equivalent test (that is log ratio test), "t" for t-test, and "z" for Z-test. With "pearson" or "spearman", the test statistic is either Pearson's correlation coefficient or Spearman's rank correlation coefficient.
paired Logical value. Depends on whether the samples are paired. Ignored if method="pearson" or method="spearman".
B Numerical value specifying the number of permutations.
yperm Optional matrix containing in each row a permutation of the class labels in binary(!) format for two-sample testing. For computation of correlation scores, the rows of yperm have to contain the appropriate values or ranks. Use this argument carefully! If yperm is specified, no other permutation will be done. Please note that the first row of yperm MUST be the input vector yin. Otherwise, the p-value calculation will be incorrect.
balance Logical value. Depends on whether balanced or unbalanced permutations should be done. Ignored if method="pearson" or method="spearman".
quant.ci Probability value for confidence lines. Lines are symmetric and denote the quant.ci-quantile of maximal absolute differences between each permutatin and the expected scores.
s0 Fudge factor for variance correction in the Z-test. Takes effect only if method="z". If s0=NULL: The fudge factor is set to the median of the pooled standard deviations.
verbose Logical value for message printing.
filtering Logical value for filtering for permutations of class labels that produce a set of complete null scores. Invokes function twilight.filtering. If yperm is specified, no filtering will be done. Note that the filtering is done on unbalanced permutations even if balance=TRUE.

Details

See vignette.

Value

Returns a twilight object consisting of a data.frame named result with variables

observed Observed test statistics.
expected Mean of order statistics of the permutation statistics.
candidate Binary vector. "1" for observations exceeding the confidence lines.
pvalue Empirical p-values from two-sided hypothesis tests.
qvalue q-values computed as described in Storey and Tibshirani (2003).
index Index of the original ordering.

Values are sorted by absolute observed scores.

Note

Additional output consists of

ci.line Quantile corresponding to quant.ci, passed for plotting.
pi0 Estimated prior probability.
call Character string of function arguments.
quant.ci Passes quant.ci for plotting.

The remaining slots are left free for function twilight.

Author(s)

Stefanie Scheid http://www.molgen.mpg.de/~scheid

References

Scheid S and Spang R (2004): A stochastic downhill search algorithm for estimating the local false discovery rate, IEEE TCBB 1(3), 98–108.

Storey JD and Tibshirani R (2003): Statistical significance for genomewide studies, PNAS 100(16), 9440–9445.

Tusher VG, Tibshirani R and Chu G (2001): Significance analysis of mircroarrays applied to the ionizing response, PNAS 98(9), 5116–5121.

See Also

twilight, plot.twilight, twilight.combi, twilight.filtering, expval

Examples

data(expval)  # twilight object created by twilight.pval on Golub data set
expval
plot(expval,which="scores")
plot(expval,which="qvalues")

[Package twilight version 1.4.0 Index]