find.a0: Computation of the Fudge Factor
In siggenes: Multiple Testing using SAM and Efron's Empirical Bayes Approaches

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

Suggests an optimal value for the fudge factor in an EBAM analysis as proposed by Efron et al. (2001).

  find.a0(data, cl, method = z.find, B = 100, delta = 0.9, 
      quan.a0 = (0:5)/5, include.zero = TRUE, 
      control = find.a0Control(), gene.names = dimnames(data)[[1]],
      rand = NA, ...)

`data`	a matrix, data frame or an ExpressionSet object. Each row of `data` (or `exprs(data)`, respectively) must correspond to a variable (e.g., a gene), and each column to a sample (i.e.\ an observation).
`cl`	a numeric vector of length `ncol(data)` containing the class labels of the samples. In the two class paired case, `cl` can also be a matrix with `ncol(data)` rows and 2 columns. If `data` is an ExpressionSet object, `cl` can also be a character string naming the column of `pData(data)` that contains the class labels of the samples. In the one-class case, `cl` should be a vector of 1's. In the two class unpaired case, `cl` should be a vector containing 0's (specifying the samples of, e.g., the control group) and 1's (specifying, e.g., the case group). In the two class paired case, `cl` can be either a numeric vector or a numeric matrix. If it is a vector, then `cl` has to consist of the integers between -1 and -n/2 (e.g., before treatment group) and between 1 and n/2 (e.g., after treatment group), where n is the length of `cl` and k is paired with -k, k=1,…,n/2. If `cl` is a matrix, one column should contain -1's and 1's specifying, e.g., the before and the after treatment samples, respectively, and the other column should contain integer between 1 and n/2 specifying the n/2 pairs of observations. In the multiclass case and if `method = cat.stat`, `cl` should be a vector containing integers between 1 and g, where g is the number of groups. For examples of how `cl` can be specified, see the manual of siggenes.
`method`	the name of a function for computing the numerator and the denominator of the test statistic of interest, and for specifying other objects required for the identification of the fudge factor. The default function `z.find` provides these objects for t- and F-statistics. It is, however, also possible to employ an user-written function. For how to write such a function, see the vignette of siggenes.
`B`	the number of permutations used in the estimation of the null distribution.
`delta`	a probability. All genes showing a posterior probability that is larger than or equal to `delta` are called differentially expressed.
`quan.a0`	a numeric vector indicating over which quantiles of the standard deviations of the genes the fudge factor a0 should be optimized.
`include.zero`	should a0 = 0, i.e. the not-modified test statistic also be a possible choice for the fudge factor?
`control`	further arguments for controlling the EBAM analysis with `find.a0`. For these arguments, see `find.a0Control`.
`gene.names`	a character vector of length `nrow(data)` containing the names of the genes. By default, the row names of `data` are used.
`rand`	integer. If specified, i.e. not `NA`, the random number generator will be set into a reproducible state.
`...`	further arguments for the function specified by `fun`. For further arguments of `fun = z.find`, see `z.find`.

The suggested choice for the fudge factor is the value of a0 that leads to the largest number of genes showing a posterior probability larger than delta.

Actually, only the genes having a posterior probability larger than delta are called differentially expressed that do not exhibit a test score less extreme than the score of a gene whose posterior probability is less than delta. So, let's say, we have done an EBAM analysis with a t-test and we have ordered the genes by their t-statistic. Let's further assume that Gene 1 to Gene 5 (i.e. the five genes with the lowest t-statistics), Gene 7 and 8, Gene 3012 to 3020, and Gene 3040 to 3051 are the only genes that show a posterior probability larger than delta. Then, Gene 1 to 5, and 3040 to 3051 are called differentially expressed, but Gene 7 and 8, and 3012 to 3020 are not called differentially expressed, since Gene 6 and Gene 3021 to 3039 show a posterior probability less than delta.

An object of class FindA0.

The numbers of differentially expressed genes can differ between find.a0 and ebam, even though the same value of the fudge factor is used, since in find.a0 the observed and permuted test scores are monotonically transformed such that the observed scores follow a standard normal distribution (if the test statistic can take both positive and negative values) and an F-distribution (if the test statistic can only take positive values) for each possible choice of the fudge factor.

Holger Schwender, holger.schw@gmx.de

Efron, B., Tibshirani, R., Storey, J.D. and Tusher, V. (2001). Empirical Bayes Analysis of a Microarray Experiment, JASA, 96, 1151-1160.

ebam, FindA0-class, find.a0Control

## Not run: 
  # Load the data of Golub et al. (1999) contained in the package multtest.
  data(golub)
  
  # golub.cl contains the class labels.
  golub.cl
  
  # Obtain the number of differentially expressed genes and the FDR for the
  # default set of values for the fudge factor.
  find.out <- find.a0(golub, golub.cl, rand = 123)
  find.out
  
  # Obtain the number of differentially expressed genes and the FDR when using
  # the t-statistic assuming equal group variances
  find.out2 <- find.a0(golub, golub.cl, var.equal = TRUE, rand = 123)
  
  # Using the Output of the first analysis with find.a0, the number of 
  # differentially expressed genes and the FDR for other values of
  # delta, e.g., 0.95, can be obtained by
  print(find.out, 0.95)
  
  # The logit-transformed posterior probabilities can be plotted by
  plot(find.out)
  
  # To avoid the logit-transformation, set logit = FALSE.
  plot(find.out, logit = FALSE)

## End(Not run)