Benchmarking Anomaly Detection Performance

Introduction

When ground truth labels are available, autoFlagR can evaluate the performance of anomaly detection algorithms using standard metrics. This is essential for validation studies and publication.

Load Required Packages

library(autoFlagR)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

Create Dataset with Known Anomalies

set.seed(123)

# Create dataset
n <- 1000
data <- data.frame(
  id = 1:n,
  feature1 = rnorm(n, 100, 15),
  feature2 = rnorm(n, 50, 10),
  feature3 = rpois(n, 5),
  category = sample(c("A", "B", "C"), n, replace = TRUE)
)

# Introduce known anomalies (ground truth)
anomaly_indices <- c(1:20, 50:55, 100:110)
data$feature1[anomaly_indices] <- data$feature1[anomaly_indices] * 5  # Extreme values
data$feature2[anomaly_indices] <- data$feature2[anomaly_indices] * 3
data$feature3[anomaly_indices] <- data$feature3[anomaly_indices] * 10

# Create ground truth labels and add to data
data$is_error <- rep(FALSE, n)
data$is_error[anomaly_indices] <- TRUE

cat("Total anomalies in ground truth:", sum(data$is_error), "\n")
#> Total anomalies in ground truth: 37

Score Anomalies with Benchmarking

# Score anomalies with ground truth for benchmarking
scored_data <- score_anomaly(
  data,
  method = "iforest",
  contamination = 0.05,
  ground_truth_col = "is_error"
)

# Check if benchmarking was performed
if (!is.null(attr(scored_data, "benchmark_metrics"))) {
  cat("Benchmarking metrics available!\n")
}
#> Benchmarking metrics available!

Extract Benchmark Metrics

# Extract benchmark metrics
metrics <- extract_benchmark_metrics(scored_data)

# Display metrics
cat("AUC-ROC:", round(metrics$auc_roc, 4), "\n")
#> AUC-ROC: 1
cat("AUC-PR:", round(metrics$auc_pr, 4), "\n")
#> AUC-PR: 0.0187
cat("Contamination Rate:", round(metrics$contamination_rate * 100, 2), "%\n")
#> Contamination Rate: 5 %
cat("\nTop-K Recall:\n")
#> 
#> Top-K Recall:
for (k_name in names(metrics$top_k_recall)) {
  k_value <- gsub("top_", "", k_name)
  recall <- metrics$top_k_recall[[k_name]]
  cat("  Top", k_value, ":", round(recall * 100, 2), "%\n")
}
#>   Top 10 : 0 %
#>   Top 50 : 0 %
#>   Top 100 : 0 %
#>   Top 500 : 0 %

Compare Different Methods

# Compare Isolation Forest vs LOF
methods <- c("iforest", "lof")
results <- list()

for (method in methods) {
  scored <- score_anomaly(
    data,
    method = method,
    contamination = 0.05,
    ground_truth_col = "is_error"
  )
  
  metrics <- extract_benchmark_metrics(scored)
  results[[method]] <- metrics
}

# Create comparison table
comparison <- data.frame(
  Method = c("Isolation Forest", "Local Outlier Factor"),
  AUC_ROC = c(results$iforest$auc_roc, results$lof$auc_roc),
  AUC_PR = c(results$iforest$auc_pr, results$lof$auc_pr),
  Top_10_Recall = c(results$iforest$top_k_recall$top_10, results$lof$top_k_recall$top_10)
)

comparison
#>                 Method   AUC_ROC     AUC_PR Top_10_Recall
#> 1     Isolation Forest 1.0000000 0.01873248           0.0
#> 2 Local Outlier Factor 0.4900508 0.04805959           0.1

Interpretation of Metrics

• AUC-ROC: Area under the ROC curve. Higher is better (max = 1.0). Measures the ability to distinguish between normal and anomalous records.

• AUC-PR: Area under the Precision-Recall curve. Higher is better (max = 1.0). More informative than AUC-ROC when classes are imbalanced.

• Top-K Recall: Percentage of true anomalies found in the top K highest-scoring records. Useful for prioritizing manual review.

Use Cases for Benchmarking

1. Algorithm Selection: Compare different methods to choose the best for your data
2. Parameter Tuning: Evaluate different contamination rates or algorithm parameters
3. Publication: Report performance metrics in research papers
4. Validation: Validate anomaly detection performance before deployment

Summary

Benchmarking with ground truth labels allows you to: - Quantitatively evaluate algorithm performance - Compare different methods and parameters - Report publication-quality metrics - Validate detection capabilities before deployment

For more details, see the Function Reference.