pensim (version 1.3.6)

opt2D: Parallelized, two-dimensional tuning of Elastic Net L1/L2 penalties

Description

This function implements parallelized two-dimensional optimization of Elastic Net penalty parameters. This is accomplished by scanning a regular grid of L1/L2 penalties, then using the top five CVL penalty combinations from this grid as starting points for the convex optimization problem.

Usage

opt2D(nsim,
      L1range = c(0.001, 100),
      L2range = c(0.001, 100),
      dofirst = "both",
      nprocessors = 1,
      L1gridsize = 10, L2gridsize = 10,
      cl = NULL,
      ...)

Value

Returns a matrix with the following columns:

L1

optimized value of the L1 penalty parameter

L2

optimized value of the L2 penalty parameter

cvl

optimized cross-validated likelihood

convergence

0 if the optimization converged, non-zero otherwise (see stats:optim for details)

fncalls

number of calls to cvl function during optimization

coef_1, coef_2, ..., coef_n

argmax coefficients for the model with this value of the tuning parameter

The matrix contains one row for each repeat of the regression.

Arguments

nsim

Number of times to repeat the simulation (around 50 is suggested)

L1range

numeric vector of length two, giving minimum and maximum constraints on the L1 penalty

L2range

numeric vector of length two, giving minimum and maximum constraints on the L2 penalty

dofirst

"L1" to optimize L1 followed by L2, "L2" to optimize L2 followed by L1, or "both" to optimize both simultaneously in a two-dimensional optimization.

nprocessors

An integer number of processors to use.

L1gridsize

Number of values of the L1 penalty in the regular grid of L1/L2 penalties

L2gridsize

Number of values of the L2 penalty in the regular grid of L1/L2 penalties

cl

Optional cluster object created with the makeCluster() function of the parallel package. If this is not set, pensim calls makeCluster(nprocessors, type="SOCK"). Setting this parameter can enable parallelization in more diverse scenarios than multi-core desktops; see the documentation for the parallel package. Note that if cl is user-defined, this function will not automatically run parallel::stopCluster() to shut down the cluster.

...

arguments passed on to optL1 and optL2 (dofirst="L1" or "L2"), or cvl (dofirst="both") functions of the penalized R package

Author

Levi Waldron et al.

Details

This function sets up a SNOW (Simple Network of Workstations) "sock" cluster to parallelize the task of repeated tunings the Elastic Net penalty parameters. Three methods are implemented, as described by Waldron et al. (2011): lambda1 followed by lambda2 (lambda1-lambda2), lambda2 followed by lambda1 (lambda2-lambda1), and lambda1 with lambda2 simultaneously (lambda1+lambda2). Tuning of the penalty parameters is done by the optL1 or optL2 functions of the penalized R package.

References

Waldron L, Pintilie M, Tsao M-S, Shepherd FA, Huttenhower C*, Jurisica I*: Optimized application of penalized regression methods to diverse genomic data. Bioinformatics 2011, 27:3399-3406. (*equal contribution)

See Also

optL1, optL2, cvl

Examples

data(beer.exprs)
data(beer.survival)

## Select just 100 genes to speed computation:
set.seed(1)
beer.exprs.sample <- beer.exprs[sample(1:nrow(beer.exprs), 100),]

## Apply an unreasonably strict gene filter here to speed computation
## time for the Elastic Net example.
gene.quant <- apply(beer.exprs.sample, 1, quantile, probs = 0.75)
dat.filt <- beer.exprs.sample[gene.quant > log2(150),]
gene.iqr <- apply(dat.filt, 1, IQR)
dat.filt <- as.matrix(dat.filt[gene.iqr > 1,])
dat.filt <- t(dat.filt)

## Define training and test sets
set.seed(9)
trainingset <- sample(rownames(dat.filt), round(nrow(dat.filt) / 2))
testset <-
  rownames(dat.filt)[!rownames(dat.filt) %in% trainingset]

dat.training <- data.frame(dat.filt[trainingset,])
pheno.training <- beer.survival[trainingset,]

library(survival)
surv.training <- Surv(pheno.training$os, pheno.training$status)

dat.test <- data.frame(dat.filt[testset,])
all.equal(colnames(dat.training), colnames(dat.test))
pheno.test <- beer.survival[testset,]
surv.test <- Surv(pheno.test$os, pheno.test$status)

set.seed(1)
##ideally set nsim=50, fold=10, but this takes 100x longer.
system.time(
  output <- opt2D(
    nsim = 1,
    L1range = c(0.1, 1),
    L2range = c(20, 1000),
    dofirst = "both",
    nprocessors = 1,
    response = surv.training,
    penalized = dat.training,
    fold = 5,
    positive = FALSE,
    standardize = TRUE
  )
)

cc <- output[which.max(output[, "cvl"]),-1:-5]
output[which.max(output[, "cvl"]), 1:5]  #small L1, large L2
sum(abs(cc) > 0)  #number of non-zero coefficients

preds.training <- as.matrix(dat.training) %*% cc
preds.training.median <- median(preds.training)
preds.training.dichot <-
  ifelse(preds.training > preds.training.median, "high risk", "low risk")
preds.training.dichot <-
  factor(preds.training.dichot[, 1], levels = c("low risk", "high risk"))
preds.test <- as.matrix(dat.test) %*% cc
preds.test.dichot <-
  ifelse(preds.test > preds.training.median, "high risk", "low risk")
preds.test.dichot <-
  factor(preds.test.dichot[, 1], levels = c("low risk", "high risk"))

coxphfit.training <- coxph(surv.training ~ preds.training.dichot)
survfit.training <- survfit(surv.training ~ preds.training.dichot)
summary(coxphfit.training)
coxphfit.test <- coxph(surv.test ~ preds.test.dichot)
survfit.test <- survfit(surv.test ~ preds.test.dichot)
summary(coxphfit.test)

(p.training <-
    signif(summary(coxphfit.training)$logtest[3], 2))  #likelihood ratio test
(hr.training <- signif(summary(coxphfit.training)$conf.int[1], 2))
(hr.lower.training <- summary(coxphfit.training)$conf.int[3])
(hr.upper.training <- summary(coxphfit.training)$conf.int[4])
par(mfrow = c(1, 2))
plot(
  survfit.training,
  col = c("black", "red"),
  conf.int = FALSE,
  xlab = "Months",
  main = "TRAINING",
  ylab = "Overall survival"
)
xmax <- par("usr")[2] - 50
text(
  x = xmax,
  y = 0.4,
  lab = paste("HR=", hr.training),
  pos = 2
)
text(
  x = xmax,
  y = 0.3,
  lab = paste("p=", p.training, "", sep = ""),
  pos = 2
)
tmp <- summary(preds.training.dichot)
text(
  x = xmax,
  y = c(0.2, 0.1),
  lab = paste(tmp, names(tmp)),
  col = 1:2,
  pos = 2
)
## Now the test set.
## in the test set,  HR=1.7 is not significant - not surprising with the
## overly strict non-specific pre-filter (IQR>1,  75th percentile > log2(150)
(p.test <-
    signif(summary(coxphfit.test)$logtest[3], 2))  #likelihood ratio test
(hr.test <- signif(summary(coxphfit.test)$conf.int[1], 2))
(hr.lower.test <- summary(coxphfit.test)$conf.int[3])
(hr.upper.test <- summary(coxphfit.test)$conf.int[4])
plot(
  survfit.test,
  col = c("black",  "red"),
  conf.int = FALSE,
  xlab = "Months",
  main = "TEST"
)
text(
  x = xmax,
  y = 0.4,
  lab = paste("HR=", hr.test),
  pos = 2
)
text(
  x = xmax,
  y = 0.3,
  lab = paste("p=", p.test, "", sep = ""),
  pos = 2
)
tmp <- summary(preds.test.dichot)
text(
  x = xmax,
  y = c(0.2, 0.1),
  lab = paste(tmp, names(tmp)),
  col = 1:2,
  pos = 2
)