Curtailing Costly Mistakes: The Feedback Loop’s Role in Elevating Testing & Reducing Defects


With ASPICE (Automotive SPICE) and ISO 26262 compliance at the forefront, car manufacturers, automotive software service companies, and autonomous driving pioneers are investing heavily in software system testing to ensure their products meet stringent standards. A pivotal metric guiding this is the Defect Escape Rate.

This article will delve into the basics of the Defect Escape Rate, its significance, and its evolution into the Defect Escape Reduction Rate.

Defect Escape Rate: The Starting Point

Defect Escape Rate, in software testing, is a critical metric that assesses the efficiency of a testing process in detecting and addressing defects throughout the development lifecycle. It measures the proportion of defects that escape detection in one phase of the development process, highlighting the importance of comprehensive testing methodologies at various stages.


Unit testing, integration testing, software requirement testing, and system qualification testing are integral components of this approach. Unit testing evaluates individual components, integration testing verifies interactions between modules, software requirement testing aligns the software with specified requirements, and system qualification testing assesses the overall system’s compliance with predefined specifications.


These testing phases collectively contribute to minimizing the Defect Escape Rate, emphasizing the significance of early detection and resolution throughout the development lifecycle. The ability to catch and address defects at each stage not only reduces the associated costs but also enhances the reliability and safety, particularly in critical domains such as automotive software.

The Escalating Cost of Defects

The cost of defects, especially in the automotive sector, is not merely a financial concern; it extends to reputation and sometimes involves human safety.

When defects slip through the early stages of development and testing and get detected later, the effort and cost required to rectify them amplify considerably. The stakes are exceptionally high in the automotive industry, in which even minor defects can have catastrophic consequences. A simple software glitch can result in accidents, injuries, or even fatalities. Therefore, early detection and rectification of defects cannot be overemphasized.

The Escalating Cost of Defects

The Defect Escape Rate Formula

The Defect Escape Rate is typically calculated using the following formula:

Defect Escape Rate = (Total Defects Identified in Phase N-1 / Defects Identified in Phase N that should have been caught in Phase N-1) × 100

This formula quantifies the extent to which defects escape the earlier phases of development and testing, highlighting the inadequacies in the testing process. The higher the Defect Escape Rate, the more defects slip through the cracks.

Defect Escape Reduction Rate: Evolving the Metric

Recognizing the gravity of high Defect Escape Rates, the automotive industry, like other sectors, has evolved its approach to quality assurance. Beyond merely identifying and measuring defects that escape, there’s a growing focus on reducing the occurrence of these escapes. This shift in perspective has led to the emergence of the Defect Escape Reduction Rate (DERR).

DERR: The Metric of Improvement

DERR quantifies the rate at which the incidence of defect escapes declines over a specified period. It reflects improvements in the testing process and code quality. Measuring DERR provides a penetrating look into the efficacy and enhancement of quality assurance measures and the development process. The formula for computing DERR is shown below:

DERR = (Change in Defect Escape Rate (DER) between two distinct points in time or project phases)

In simpler terms, DERR is like the “acceleration” in software quality metrics. Just as acceleration measures the rate of change in speed over time, DERR measures the rate of change in defect escape reduction over time.

Ideal & Realistic Improvement Curve

A new team embarking on a novel development process often experiences a learning curve. They adapt to new tools, techniques, and the product itself. This adaptation phase may initially result in a relatively flat or even rising DERR. As the team becomes adept with the technology, tools, and processes, defect detection improves as the processes mature, leading to a reduced defect escape rate. Insights from early project stages are implemented to foster improvement.

The curve begins to slope downward, indicating improvements in the defect escape rate. However, it’s important to note that this improvement might slow down as the team encounters diminishing returns on their refinement efforts. Eventually, the team reaches a point at which processes are optimized, and defect escapes are minimized. Further improvements become incremental and more challenging, resulting in a plateaued curve with minor ups and downs.

The Role of a Feedback Loop

The feedback loop, at the heart of DERR measurement and improvement, is crucial in the journey to reduce defect escapes. It’s the mechanism through which lessons learned from defect escapes are incorporated into the testing process to prevent similar issues in the future.

Implementing the Feedback Loop for DERR Improvement

Listed below are feedback loops that can be implemented for DERR improvement:

Root Cause Analysis

Root cause analysis is understanding why a defect occurred in the first place. It goes beyond surface-level fixes and seeks to identify systemic issues contributing to defects. In the automotive software industry, root cause analysis can distinguish between a safe, dependable system and one riddled with vulnerabilities. Here are some critical steps to practical root cause analysis:

  1. Choose a tool to categorize and tag defects based on where they were found and where they should have been detected.
  2. Educate team members on how to classify defects and the importance of understanding their origins.
  3. Record defects as they arise, noting their potential causes and related tasks.
  4. For every defect that wasn’t caught on time, ask “why?” Use simple techniques like the “5 Whys” to get to the bottom of it.
  5. Gather all the defects identified every week or two. Determine which should’ve been caught earlier to calculate the Internal Escape Defect Rate.
  6. Plot your defect rate over time. Use symbols or colors to highlight patterns or recurring issues.
  7. Make changes to your process based on what you learn from the root causes. Watch how these changes impact future defect rates.
  8. Regularly revisit your defect tracking and analysis methods—iterate based on what’s working best for your team.

Implementing the Feedback Loop for DERR Improvement

Test automation uses automated scripts and tools to test software applications. It can be a game-changer in automotive software, especially regarding ensuring quality and reliability.

Here’s why test automation is crucial in the realm of ASPICE ISO 26262 software system testing:

  1. Can ensure that regressions are caught early in development. As changes are made to the software, automated tests can be executed rapidly to check if existing functionality still works as expected.
  2. Human testers may introduce variability into the testing process. Automated tests are consistent and reproducible, ensuring that the same tests are conducted in the same way every time.
  3. Allows for the rapid execution of tests, saving time and resources. This is particularly important in the fast-paced automotive industry, in which software updates and releases are frequent.
  4. Can perform a wide range of tests, from unit tests to integration tests and system tests. It can also simulate real-world scenarios to ensure that software functions correctly in various conditions.

A critical component of CI/CD pipelines, where software changes are automatically built, tested, and deployed. This ensures that defects are caught early and that software releases are reliable.

The Feedback Loop: Quick and Actionable Insights

A tight feedback loop between developers and testers is essential in the quest for quality and reliability. It ensures that defects are addressed promptly, reducing the chances of similar issues in subsequent releases. In the context of ASPICE ISO 26262 software system testing, this feedback loop is the bridge that connects the identification of defects to their resolution.

The key elements of a robust feedback loop in the automotive software industry include:

  1. Developer involvement
  2. Testing as a priority
  3. Education and training
  4. Knowledge and training

Training and Skill Development

Periodic training sessions can be instrumental in ensuring the team is updated with the latest testing methodologies and tools. When teams understand the importance of their role and are equipped with the right tools, the number of defects can decrease.

Knowledge Sharing

Encourage teams to share knowledge about recurring defects and their solutions. This can be done through documentation, tech talks, or informal discussions. Sharing knowledge helps prevent the recurrence of similar issues and contributes to DERR improvement.

Complexity is Not an Excuse

The complexity of modern automotive software systems should not deter us from striving for excellence. While the automotive industry faces unique challenges, particularly regarding safety-critical applications, effective metrics, and continuous improvement processes can help navigate this complexity and reduce defect escapes.

Business Impact

Defects in critical safety software, such as that used in autonomous driving systems, can have direct business implications. A high defect escape rate could mean potential recalls, system downtimes, or life-threatening situations. From a financial standpoint, defects caught later in the development cycle or post-production are notoriously more expensive. DERR helps in quantifying and mitigating these business risks.

Empirical Evidence Over Anecdotal Arguments

In the context of automotive software development, relying on empirical evidence rather than anecdotal arguments is crucial. Numerous studies have shown that when properly managed and paired with effective methodologies, metrics like the Defect Escape Rate and its reduction can significantly improve software quality and reliability. This evidence-based approach is a cornerstone of quality assurance in the automotive sector.

The Importance of a Feedback Loop

In automotive software development, the journey to reduce defect escapes is an ongoing endeavor. The Defect Escape Rate and its evolution into the Defect Escape Reduction Rate serve as critical metrics that guide this journey. The feedback loop plays a pivotal role in this quest for excellence. It ensures that lessons learned from defect escapes are not lost but are incorporated into the testing process to prevent similar issues in the future.

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