goodTuring: Good-Turing Frequency Estimation

In OliverVoogd/edgeR: Empirical Analysis of Digital Gene Expression Data in R

Good-Turing Frequency Estimation

Description

Non-parametric empirical Bayes estimates of the frequencies of observed (and unobserved) species.

Usage

goodTuring(x, conf=1.96)
goodTuringPlot(x)
goodTuringProportions(counts)

Arguments

x	numeric vector of non-negative integers, representing the observed frequency of each species.
conf	confidence factor, as a quantile of the standard normal distribution, used to decide for what values the log-linear relationship between frequencies and frequencies of frequencies is acceptable.
counts	matrix of counts

Details

Observed counts are assumed to be Poisson distributed. Using an non-parametric empirical Bayes strategy, the algorithm evaluates the posterior expectation of each species mean given its observed count. The posterior means are then converted to proportions. In the empirical Bayes step, the counts are smoothed by assuming a log-linear relationship between frequencies and frequencies of frequencies. The fundamentals of the algorithm are from Good (1953). Gale and Sampson (1995) proposed a simplied algorithm with a rule for switching between the observed and smoothed frequencies, and it is Gale and Sampson's simplified algorithm that is implemented here. The number of zero values in x is not used as part of the algorithm, but is returned by this function.

Sampson gives a C code version on his webpage at http://www.grsampson.net/RGoodTur.html which gives identical results to this function.

goodTuringPlot plots log-probability (i.e., log frequencies of frequencies) versus log-frequency.

goodTuringProportions runs goodTuring on each column of data, then uses the results to predict the proportion of each gene in each library.

Value

goodTuring returns a list with components

count	observed frequencies, i.e., the unique positive values of x
n	frequencies of frequencies
n0	frequency of zero, i.e., number of zeros found in x
proportion	estimated proportion of each species given its count
P0	estimated combined proportion of all undetected species

goodTuringProportions returns a matrix of proportions of the same size as counts.

Author(s)

Aaron Lun and Gordon Smyth, adapted from Sampson's C code from http://www.grsampson.net/RGoodTur.html

References

Gale, WA, and Sampson, G (1995). Good-Turing frequency estimation without tears. Journal of Quantitative Linguistics 2, 217-237.

Good, IJ (1953). The population frequencies of species and the estimation of population parameters. Biometrika 40, 237-264.

Examples

#  True means of observed species
lambda <- rnbinom(10000,mu=2,size=1/10)
lambda <- lambda[lambda>1]

#  Oberved frequencies
Ntrue <- length(lambda)
x <- rpois(Ntrue, lambda=lambda)
freq <- goodTuring(x)
goodTuringPlot(x)

OliverVoogd/edgeR documentation built on July 28, 2022, 10:13 p.m.