multRepl: Multiplicative simple replacement

In zCompositions: Treatment of Zeros, Left-Censored and Missing Values in Compositional Data Sets

Multiplicative simple replacement

Description

This function implements non-parametric multiplicative simple imputation of left-censored (e.g. values below detection limit, rounded zeros) and missing data in compositional data sets.

Usage

multRepl(X, label = NULL, dl = NULL, frac = 0.65, imp.missing = FALSE, closure = NULL,
            z.warning=0.8, z.delete = TRUE, delta = NULL)

Arguments

X	Compositional vector (numeric class) or data set (matrix or data.frame class).
label	Unique label (numeric or character) used to denote zeros/unobserved values in X.
dl	Numeric vector or matrix of detection limits/thresholds. These must be given on the same scale as X. If NULL the column minima are used as thresholds.
frac	Fraction of the detection limit/threshold used for imputation (default = 0.65, expressed as a proportion).
imp.missing	If TRUE then unobserved values identified by label are treated as missing data (default = FALSE).
closure	Closure value used to add a residual part for imputation (see below).
z.warning	Threshold used to identify individual rows or columns including an excess of zeros/unobserved values (to be specify in proportions, default z.warning=0.8).
z.delete	Logical value. If set to TRUE, rows/columns identified by z.warning are omitted in the imputed data set. Otherwise, the function stops in error when rows/columns are identified by z.warning (default z.delete=TRUE).
delta	This argument has been deprecated and replaced by frac (see package's NEWS for details).

Details

This function imputes left-censored compositional values by a given fraction frac of the corresponding limit of detection and applies a multiplicative adjustment to preserve the multivariate compositional properties of the samples. It allows for either single (vector form) or multiple (matrix form, same size as X) limits of detection by component. Any threshold value can be set for non-censored elements (e.g. use 0 if no threshold for a particular column or element of the data matrix).

Missing data imputation: missing data can be imputed by setting imp.missing = TRUE. They are replaced by the estimated column geometric mean from observed values. The non-missing parts in the composition are applied multiplicative adjustment. The argument dl and frac are ignored and X is require to be a data matrix in this case.

Note: negative values can be generated when unobserved components are a large portion of the composition, which is more likely for missing data (e.g in major chemical elements) and non-closed compositions. A workaround is to add a residual filling the gap up to the closure/total when possible. This is done internally when a value for closure is specified (e.g. closure=10^6 if ppm or closure=100 if percentages). The residual is discarded after imputation.

This function produces an imputed data set on the same scale as the input data set. If X is not closed to a constant sum, then the results are adjusted to provide a compositionally equivalent data set, expressed in the original scale, which leaves the absolute values of the observed components unaltered. Note that this adjustment only applies to data sets and not when a single composition is entered. In this latter case, the composition is treated as a closed vector.

Value

A data.frame object containing the imputed compositional vector or data set expressed in the original scale.

References

Martin-Fernandez JA, Barcelo-Vidal C, Pawlowsky-Glahn V. Dealing with zeros and missing values in compositional data sets using nonparametric imputation. Mathematical Geology 2003; 35: 253-78.

Palarea-Albaladejo J, Martin-Fernandez JA. Values below detection limit in compositional chemical data. Analytica Chimica Acta 2013; 764: 32-43.

Palarea-Albaladejo J. and Martin-Fernandez JA. zCompositions – R package for multivariate imputation of left-censored data under a compositional approach. Chemometrics and Intelligence Laboratory Systems 2015; 143: 85-96.

See Also

zPatterns, lrEM, lrSVD, lrDA, multLN, multKM, cmultRepl

Examples

# A compositional vector (NA indicates nondetect)
y <- c(0.6,NA,0.25,0.03,0.12,NA)
dl <- c(0,0.01,0,0,0,0.005)
# Using the default frac = 0.65
yr <- multRepl(y,label=NA,dl=dl)
round(yr,4)

# Data set closed to 100 (percentages, common dl = 1%)
X <- matrix(c(26.91,8.08,12.59,31.58,6.45,14.39,
              39.73,26.20,0.00,15.22,6.80,12.05,
              10.76,31.36,7.10,12.74,31.34,6.70,
              10.85,46.40,31.89,10.86,0.00,0.00,
              7.57,11.35,30.24,6.39,13.65,30.80,
              38.09,7.62,23.68,9.70,20.91,0.00,
              27.67,7.15,13.05,32.04,6.54,13.55,
              44.41,15.04,7.95,0.00,10.82,21.78,
              11.50,30.33,6.85,13.92,30.82,6.58,
              19.04,42.59,0.00,38.37,0.00,0.00),byrow=TRUE,ncol=6)
              
X_multRepl <- multRepl(X,label=0,dl=rep(1,6))

# Multiple limits of detection by component
mdl <- matrix(0,ncol=6,nrow=10)
mdl[2,] <- rep(1,6)
mdl[4,] <- rep(0.75,6)
mdl[6,] <- rep(0.5,6)
mdl[8,] <- rep(0.5,6)
mdl[10,] <- c(0,0,1,0,0.8,0.7)

X_multRepl2 <- multRepl(X,label=0,dl=mdl)

# Non-closed compositional data set
data(LPdata) # data (ppm/micrograms per gram)
dl <- c(2,1,0,0,2,0,6,1,0.6,1,1,0,0,632,10) # limits of detection (0 for no limit)
LPdata_multRepl <- multRepl(LPdata,label=0,dl=dl)

# Two subsets of limits of detection
data(LPdata)
dl <- c(2,1,0,0,2,0,6,1,0.6,1,1,0,0,632,10) # limits of detection (0 for no limit)
 # DLs for first 50 samples of LPdata
dl1 <- matrix(rep(1,50),ncol=1)%*%dl
 # DLs for last 46 samples of LPdata
dl2 <- matrix(rep(1,46),ncol=1)%*%c(1,0.5,0,0,2.5,0,5.5,0.75,0.3,1.5,1,0,0,600,8) 

mdl <- rbind(dl1,dl2)
LPdata_multRepl2 <- multRepl(LPdata,label=0,dl=mdl)

# Data set with missing values closed to 100 (percentages)
X <- matrix(c(10.47,8.58,59.72,19.30,1.93,
              12.13,7.44,62.87,16.37,1.19,
                 NA,7.30,75.91,16.79,NA,
               9.77,7.80,65.68,14.78,1.97,
              10.79,9.55,65.87,12.41,1.38,
              14.54,8.18,64.55,12.73,NA,
              12.28,7.58,66.01,12.93,1.20,
              28.09,22.92,NA,40.11,8.88,
               7.02,6.30,75.65,11.03,NA),byrow=TRUE,ncol=5)
              
X_multReplMiss <- multRepl(X,label=NA,imp.missing=TRUE)

# Non-closed compositional data set
data(LPdata) # (in ppm units)
# Treating zeros as missing data for illustration purposes only
LPdata_multReplMiss <- multRepl(LPdata,label=0,imp.missing=TRUE)
# Negative values generated (see e.g. K and Rb in sample #60)

# Workaround: use residual part to fill up the gap to 10^6 for imputation
LPdata_multReplMiss2 <- multRepl(LPdata,label=0,imp.missing=TRUE,closure=10^6)

zCompositions documentation built on June 22, 2024, 9:46 a.m.