ROC

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ROC curve

• Binary case:
• Y = true positive rate = sensitivity,
• X = false positive rate = 1-specificity = 假陽性率
• Area under the curve AUC from the wikipedia: the probability that a classifier will rank a randomly chosen positive instance higher than a randomly chosen negative one (assuming 'positive' ranks higher than 'negative').
$\displaystyle{ A = \int_{\infty}^{-\infty} \mbox{TPR}(T) \mbox{FPR}'(T) \, dT = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} I(T'\gt T)f_1(T') f_0(T) \, dT' \, dT = P(X_1 \gt X_0) }$
where $\displaystyle{ X_1 }$ is the score for a positive instance and $\displaystyle{ X_0 }$ is the score for a negative instance, and $\displaystyle{ f_0 }$ and $\displaystyle{ f_1 }$ are probability densities as defined in previous section.
• Interpretation of the AUC. A small toy example (n=12=4+8) was used to calculate the exact probability $\displaystyle{ P(X_1 \gt X_0) }$ (4*8=32 all combinations).
• It is a discrimination measure which tells us how well we can classify patients in two groups: those with and those without the outcome of interest.
• Since the measure is based on ranks, it is not sensitive to systematic errors in the calibration of the quantitative tests.
• The AUC can be defined as The probability that a randomly selected case will have a higher test result than a randomly selected control.
• Plot of sensitivity/specificity (y-axis) vs cutoff points of the biomarker
• The Mann-Whitney U test statistic (or Wilcoxon or Kruskall-Wallis test statistic) is equivalent to the AUC (Mason, 2002)
• The p-value of the Mann-Whitney U test can thus safely be used to test whether the AUC differs significantly from 0.5 (AUC of an uninformative test).
• Calculate AUC by hand. AUC is equal to the probability that a true positive is scored greater than a true negative.
• See the uROC() function in <functions.R> from the supplementary of the paper (need access right) Bivariate Marker Measurements and ROC Analysis Wang 2012. Let $\displaystyle{ n_1 }$ be the number of obs from X1 and $\displaystyle{ n_0 }$ be the number of obs from X0. X1 and X0 are the predict values for data from group 1 and 0. $\displaystyle{ TP_i=Prob(X_1\gt X_{0i})=\sum_j (X_{1j} \gt X_{0i})/n_1, ~ FP_i=Prob(X_0\gt X_{0i}) = \sum_j (X_{0j} \gt X_{0i}) / n_0 }$. We can draw a scatter plot or smooth.spline() of TP(y-axis) vs FP(x-axis) for the ROC curve.
uROC <- function(marker, status)   ### ROC function for univariate marker ###
{
x <- marker
cat(paste("\n", sum(bad), "records with missing values dropped. \n"))
no_case <- sum(status==1)
no_control <- sum(status==0)
TP <- rep(0, no_control)
FP <- rep(0, no_control)
for (i in 1: no_control){
TP[i] <- sum(x[status==1]>x[status==0][i])/no_case
FP[i] <- sum(x[status==0]>x[status==0][i])/no_control
}
list(TP = TP, FP = FP)
}

Survival data

'Survival Model Predictive Accuracy and ROC Curves' by Heagerty & Zheng 2005

• Recall Sensitivity= $\displaystyle{ P(\hat{p_i} \gt c | Y_i=1) }$, Specificity= $\displaystyle{ P(\hat{p}_i \le c | Y_i=0 }$), $\displaystyle{ Y_i }$ is binary outcomes, $\displaystyle{ \hat{p}_i }$ is a prediction, $\displaystyle{ c }$ is a criterion for classifying the prediction as positive ($\displaystyle{ \hat{p}_i \gt c }$) or negative ($\displaystyle{ \hat{p}_i \le c }$).
• For survival data, we need to use a fixed time/horizon (t) to classify the data as either a case or a control. Following Heagerty and Zheng's definition in Survival Model Predictive Accuracy and ROC Curves (Incident/dynamic) 2005, Sensitivity(c, t)= $\displaystyle{ P(M_i \gt c | T_i = t) }$, Specificity= $\displaystyle{ P(M_i \le c | T_i \gt t }$) where M is a marker value or $\displaystyle{ Z^T \beta }$. Here sensitivity measures the expected fraction of subjects with a marker greater than c among the subpopulation of individuals who die at time t, while specificity measures the fraction of subjects with a marker less than or equal to c among those who survive beyond time t.
• The AUC measures the probability that the marker value for a randomly selected case exceeds the marker value for a randomly selected control
• ROC curves are useful for comparing the discriminatory capacity of different potential biomarkers.

Confusion matrix, Sensitivity/Specificity/Accuracy

 Predict 1 0 True 1 TP FN Sens=TP/(TP+FN)=Recall FNR=FN/(TP+FN) 0 FP TN Spec=TN/(FP+TN), 1-Spec=FPR PPV=TP/(TP+FP) FDR=FP/(TP+FP) NPV=TN/(FN+TN) N = TP + FP + FN + TN
• Sensitivity 敏感度 = TP / (TP + FN) = Recall
• Specificity 特異度 = TN / (TN + FP)
• Accuracy = (TP + TN) / N
• False discovery rate FDR = FP / (TP + FP)
• False negative rate FNR = FN / (TP + FN)
• False positive rate FPR = FP / (FP + TN)
• True positive rate = TP / (TP + FN)
• Positive predictive value (PPV) = TP / # positive calls = TP / (TP + FP) = 1 - FDR
• Negative predictive value (NPV) = TN / # negative calls = TN / (FN + TN)
• Prevalence 盛行率 = (TP + FN) / N.
• Note that PPV & NPV can also be computed from sensitivity, specificity, and prevalence:
$\displaystyle{ \text{PPV} = \frac{\text{sensitivity} \times \text{prevalence}}{\text{sensitivity} \times \text{prevalence}+(1-\text{specificity}) \times (1-\text{prevalence})} }$
$\displaystyle{ \text{NPV} = \frac{\text{specificity} \times (1-\text{prevalence})}{(1-\text{sensitivity}) \times \text{prevalence}+\text{specificity} \times (1-\text{prevalence})} }$

genefilter package and rowpAUCs function

• rowpAUCs function in genefilter package. The aim is to find potential biomarkers whose expression level is able to distinguish between two groups.
# source("http://www.bioconductor.org/biocLite.R")
# biocLite("genefilter")
library(Biobase) # sample.ExpressionSet data
data(sample.ExpressionSet)

library(genefilter)
r2 = rowpAUCs(sample.ExpressionSet, "sex", p=0.1)
plot(r2) # first gene, asking specificity = .9

r2 = rowpAUCs(sample.ExpressionSet, "sex", p=1.0)
plot(r2) # it won't show pAUC

r2 = rowpAUCs(sample.ExpressionSet, "sex", p=.999)
plot(r2) # pAUC is very close to AUC now

Comparison of two AUCs

• Statistical Assessments of AUC. This is using the pROC::roc.test function.
• prioritylasso. It is using roc(), auc(), roc.test(), plot.roc() from the pROC package. The calculation based on the training data is biased so we need to report the one based on test data.

Confidence interval of AUC

How to get an AUC confidence interval. pROC package was used.

AUC can be a misleading measure of performance

AUC is high but precision is low (i.e. FDR is high). https://twitter.com/michaelhoffman/status/1398380674206285830?s=09.

Class comparison problem

• compcodeR: RNAseq data simulation, differential expression analysis and performance comparison of differential expression methods
• Polyester: simulating RNA-seq datasets with differential transcript expression, github, HTML