imputation: Quantitative mass spectrometry data imputation

In rformassspectrometry/MsCoreUtils: Core Utils for Mass Spectrometry Data

Quantitative mass spectrometry data imputation

Description

The impute_matrix function performs data imputation on matrix objects instance using a variety of methods (see below).

Users should proceed with care when imputing data and take precautions to assure that the imputation produces valid results, in particular with naive imputations such as replacing missing values with 0.

Usage

impute_matrix(x, method, FUN, ...)

imputeMethods()

impute_neighbour_average(x, k = min(x, na.rm = TRUE), MARGIN = 1L)

impute_knn(x, MARGIN = 1L, ...)

impute_mle(x, MARGIN = 2L, ...)

impute_bpca(x, MARGIN = 1L, ...)

impute_RF(x, MARGIN = 2L, ...)

impute_mixed(x, randna, mar, mnar, MARGIN = 1L, ...)

impute_min(x)

impute_MinDet(x, q = 0.01, MARGIN = 2L)

impute_MinProb(x, q = 0.01, sigma = 1, MARGIN = 2L)

impute_QRILC(x, sigma = 1, MARGIN = 2L)

impute_zero(x)

impute_with(x, val)

impute_fun(x, FUN, MARGIN = 1L, ...)

getImputeMargin(fun)

Arguments

x	A matrix or an HDF5Matrix object to be imputed.
method	character(1) defining the imputation method. See imputeMethods() for available ones.
FUN	A user-provided function that takes a matrix as input and returns an imputed matrix of identical dimensions.
...	Additional parameters passed to the inner imputation function.
k	numeric(1) providing the imputation value used for the first and last samples if they contain an NA. The default is to use the smallest value in the data.
MARGIN	integer(1) defining the margin along which to apply imputation, with 1L for rows and 2L for columns. The default value will depend on the imputation method. Use getImputeMargin(fun) to get the default margin of imputation function fun. If the function doesn't take a margin argument, NA is returned.
randna	logical of length equal to nrow(object) defining which rows are missing at random. The other ones are considered missing not at random. Only relevant when methods is mixed.
mar	Imputation method for values missing at random. See method above.
mnar	Imputation method for values missing not at random. See method above.
q	numeric(1) indicating the quantile to be used to estimate the minimum in MinDet and MinProb. Default is 0.01.
sigma	numeric(1) controling the standard deviation of the MNAR distribution in MinProb and QRILC. Default is 1.
val	numeric(1) used to replace all missing values.
fun	The imputation function to get the default margin from.

Value

A matrix of same class as x with dimensions dim(x).

Types of missing values

There are two types of mechanisms resulting in missing values in LC/MSMS experiments.

• Missing values resulting from absence of detection of a feature, despite ions being present at detectable concentrations. For example in the case of ion suppression or as a result from the stochastic, data-dependent nature of the DDA MS acquisition method. These missing value are expected to be randomly distributed in the data and are defined, in statistical terms, as missing at random (MAR) or missing completely at random (MCAR).

• Biologically relevant missing values resulting from the absence or the low abundance of ions (i.e. below the limit of detection of the instrument). These missing values are not expected to be randomly distributed in the data and are defined as missing not at random (MNAR).

MNAR features should ideally be imputed with a left-censor method, such as QRILC below. Conversely, it is recommended to use hot deck methods such nearest neighbours, Bayesian missing value imputation or maximum likelihood methods when values are missing at random.

Imputing by rows or columns

We assume that the input matrix x contains features along the rows and samples along the columns, as is generally the case in omics data analysis. When performing imputation, the missing values are taken as a feature-specific property: feature x is missing because it is absent (in a sample or group), or because it was missed during acquisition (not selected during data dependent acquisition) or data processing (not identified or with an identification score below a chosen false discovery threshold). As such, imputation is by default performed at the feature level. In some cases, such as imputation by zero or a global minimum value, it doesn't matter. In other cases, it does matter very much, such as for example when using the minimum value computed for each margin (i.e. row or column) as in the MinDet method (see below) - do we want to use the minimum of the sample or of that feature? KNN is another such example: do we consider the most similar features to impute a feature with missing values, or the most similar samples to impute all missing in a sample.

The MARGIN argument can be used to change the imputation margin from features/rows (MARGIN = 1) to samples/columns (MARGIN = 2). Different imputations will have different default values, and changing this parameter can have a major impact on imputation results and downstream results.

Imputation methods

Currently, the following imputation methods are available.

• MLE: Maximum likelihood-based imputation method using the EM algorithm. The impute_mle() function relies on norm::imp.norm(). function. See norm::imp.norm() for details and additional parameters. Note that here, ... are passed to the norm::em.norm() function, rather to the actual imputation function imp.norm.

• bpca: Bayesian missing value imputation are available, as implemented in the pcaMethods::pca() function. See pcaMethods::pca() for details and additional parameters.

• RF: Random Forest imputation, as implemented in the missForest::missForest function. See missForest::missForest()] for details and additional parameters.

• knn: Nearest neighbour averaging, as implemented in the impute::impute.knn function. See impute::impute.knn()] for details and additional parameters.

• QRILC: A missing data imputation method that performs the imputation of left-censored missing data using random draws from a truncated distribution with parameters estimated using quantile regression. The impute_QRILC() function calls imputeLCMD::impute.QRILC() from the imputeLCMD package.

• MinDet: Performs the imputation of left-censored missing data using a deterministic minimal value approach. Considering a expression data with n samples and p features, for each sample, the missing entries are replaced with a minimal value observed in that sample. The minimal value observed is estimated as being the q-th quantile (default q = 0.01) of the observed values in that sample. The implementation in based on the imputeLCMD::impute.MinDet() function.

• MinProb: Performs the imputation of left-censored missing data by random draws from a Gaussian distribution centred to a minimal value. Considering an expression data matrix with n samples and p features, for each sample, the mean value of the Gaussian distribution is set to a minimal observed value in that sample. The minimal value observed is estimated as being the q-th quantile (default q = 0.01) of the observed values in that sample. The standard deviation is estimated as the median of the feature (or sample) standard deviations. Note that when estimating the standard deviation of the Gaussian distribution, only the peptides/proteins which present more than 50\ values are considered. The impute_MinProb() function calls imputeLCMD::impute.MinProb() from the imputeLCMD package.

• min: Replaces the missing values with the smallest non-missing value in the data.

• zero: Replaces the missing values with 0.

• mixed: A mixed imputation applying two methods (to be defined by the user as mar for values missing at random and mnar for values missing not at random, see example) on two MCAR/MNAR subsets of the data (as defined by the user by a randna logical, of length equal to nrow(object)).

• nbavg: Average neighbour imputation for fractions collected along a fractionation/separation gradient, such as sub-cellular fractions. The method assumes that the fraction are ordered along the gradient and is invalid otherwise.

  Continuous sets NA value at the beginning and the end of the quantitation vectors are set to the lowest observed value in the data or to a user defined value passed as argument k. Then, when a missing value is flanked by two non-missing neighbouring values, it is imputed by the mean of its direct neighbours.

• with: Replaces all missing values with a user-provided value.

• none: No imputation is performed and the missing values are left untouched. Implemented in case one wants to only impute value missing at random or not at random with the mixed method.

The imputeMethods() function returns a vector with valid imputation method names. Use getImputeMargin() to get the default margin for each imputation function.

Author(s)

Laurent Gatto

References

Olga Troyanskaya, Michael Cantor, Gavin Sherlock, Pat Brown, Trevor Hastie, Robert Tibshirani, David Botstein and Russ B. Altman, Missing value estimation methods for DNA microarrays Bioinformatics (2001) 17 (6): 520-525.

Oba et al., A Bayesian missing value estimation method for gene expression profile data, Bioinformatics (2003) 19 (16): 2088-2096.

Cosmin Lazar (2015). imputeLCMD: A collection of methods for left-censored missing data imputation. R package version 2.0. http://CRAN.R-project.org/package=imputeLCMD.

Lazar C, Gatto L, Ferro M, Bruley C, Burger T. Accounting for the Multiple Natures of Missing Values in Label-Free Quantitative Proteomics Data Sets to Compare Imputation Strategies. J Proteome Res. 2016 Apr 1;15(4):1116-25. doi: 10.1021/acs.jproteome.5b00981. PubMed PMID:26906401.

Examples

## test data
set.seed(42)
m <- matrix(rlnorm(60), 10)
dimnames(m) <- list(letters[1:10], LETTERS[1:6])
m[sample(60, 10)] <- NA

## available methods
imputeMethods()

impute_matrix(m, method = "zero")

impute_matrix(m, method = "min")

impute_matrix(m, method = "knn")

## same as impute_zero
impute_matrix(m, method = "with", val = 0)

## impute with half of the smalles value
impute_matrix(m, method = "with",
              val = min(m, na.rm = TRUE) * 0.5)

## all but third and fourth features' missing values
## are the result of random missing values
randna <- rep(TRUE, 10)
randna[c(3, 9)] <- FALSE

impute_matrix(m, method = "mixed",
              randna = randna,
              mar = "knn",
              mnar = "min")


## user provided (random) imputation function
random_imp <- function(x) {
   m <- mean(x, na.rm = TRUE)
   sdev <- sd(x, na.rm = TRUE)
   n <- sum(is.na(x))
   x[is.na(x)] <- rnorm(n, mean = m, sd = sdev)
   x
}

impute_matrix(m, FUN = random_imp)

## get the default margin
getImputeMargin(impute_knn) ## default imputes along features

getImputeMargin(impute_mle) ## default imputes along samples

getImputeMargin(impute_zero) ## NA: no margin here

## default margin for all MsCoreUtils::impute_* functions
sapply(ls("package:MsCoreUtils", pattern = "impute_"), getImputeMargin)

rformassspectrometry/MsCoreUtils documentation built on Oct. 24, 2024, 1:52 p.m.