Dependent Failure Analysis (DFA)
You learn to prove that redundant channels and ASIL decomposition really are independent, running the six-step ISO 26262-9 Clause 7 method to hunt dependent failure initiators, coupling factors, and cascading paths across hardware and software.
- Chapters
- 10
- Chapters
- Step Method
- 6
- Step Method
- Coupling Categories
- 4
- Coupling Categories
- ISO 26262-9
- Clause 7
- ISO 26262-9
Why it pays for itself
Prove independence instead of assuming it
Redundancy and ASIL decomposition claims collapse when one root cause defeats both channels. The six-step DFA method turns those assumptions into a validated independence argument backed by evidence.
Hunt initiators with a checklist
A category-by-category coupling-factor checklist - proximity, shared power and ground, shared memory and communication, common IP and tooling - drives complete coverage so no plausible dependent failure initiator slips through.
Close a real DFI register
The dual-channel brake-by-wire worked example moves from stated independence claims to a scored DFI register with mitigations, open items, and the evidence needed to close each one.
What you’ll be able to do
Validate independence claims
Decide whether ASIL decomposition and redundancy claims hold by proving no single root cause bridges the channels.
Identify dependent failure initiators
Systematically surface the root-cause events and coupling factors that link elements you assumed were independent.
Run the six-step method
Apply the structured DFA process from element identification through coupling assessment to verified closure.
Select effective mitigations
Choose physical separation, diversity, partitioning, and monitoring measures that actually break the coupling you found.
Place DFA across the lifecycle
Schedule DFA iterations early and at each V-model phase so couplings are caught when fixes are cheap.
Separate DFA, FFI, and CCF
Use each term correctly in safety documentation and connect them into one coherent independence argument.
Chapter by chapter
- 01
Why DFA Matters
Independence and redundancy claims collapse the moment a single root cause can defeat both channels at once, which is exactly what DFA exists to catch.
- Statistical independence is an assumption, not a physical guarantee
- Backs ASIL decomposition, freedom from interference, and redundancy claims
- Start at concept phase to avoid costly late rework
- 02
Common-Cause vs Cascading
Two distinct patterns by which one initiating event defeats several safety-relevant elements, with the normative definitions that keep your documentation precise.
- Common-cause failure hits multiple elements from one shared root
- Cascading failure propagates from one element to the next
- Automotive examples drawn from real ECU and sensor architectures
- 03
Coupling Factors and DFI
The analytical core of DFA, where you identify dependent failure initiators and the coupling factors (shared power, clock, memory, environment) that connect supposedly independent elements.
- A DFI is the root-cause event that triggers multiple elements
- Coupling categories span resource, environment, communication, and design
- Trace each DFI through its pathway to a dependent failure
- 04
DFA in the Lifecycle
DFA runs iteratively across the V-model rather than as a single gate, with defined inputs and outputs at the concept, hardware, and software phases.
- Performed from concept through hardware and software design
- Inputs include architecture, independence claims, and FMEDA data
- Rigor scales with the ASIL of the safety goal being protected
- 05
The DFA Method
A structured six-step process that turns architecture documentation into a validated independence argument, complete with a reusable worksheet and the pitfalls to avoid.
- Six steps from element identification to verified closure
- Walkthrough against a coupling-factor taxonomy
- DFI worksheet captures initiator, coupling, severity, and mitigation
- 06
Coupling-Factor Checklist
A category-by-category checklist (proximity, shared power and ground, shared memory and communication, common IP and tooling) so no plausible DFI path slips through.
- Four ordered categories covering physical to design coupling
- Each item resolved as mitigated, accepted, or open
- Drives complete coverage during the DFI hunt
- 07
DFA for Hardware
Hardware is where DFIs are most tangible, so this chapter covers shared power, shared die, and shared ground, plus a dual-core lockstep case study and a mitigation hierarchy.
- Shared supply, die, and ground plane as common-cause sources
- Dual-core lockstep common-cause concerns examined
- Mitigation hierarchy from separation to monitoring
- 08
DFA for Software
Software coupling rarely shows up in schematics, so this chapter exposes shared memory, shared operating system, and shared libraries that can silently break independence.
- Shared memory, OS, and libraries defeat software independence
- Freedom from interference across timing, memory, and exchange
- Mixed-ASIL coexistence triggers a DFA per Clause 7.4.11
- 09
Worked Example
A complete DFA walkthrough for a dual-channel brake-by-wire architecture, moving from stated independence claims to a fully closed DFI register.
- Redundant sensor pair and dual ECU under analysis
- Candidate DFI register with severity and coupling
- Mitigation details and a clear open-item closure path
- 10
DFA vs FFI vs CCF
DFA, freedom from interference, common-cause failure, and cascading failure are four distinct but linked terms, and this chapter pins down each one for correct safety documentation.
- Precise definitions with their ISO 26262 term references
- How the four concepts relate without overlapping
- Avoids the terminology mix-ups that weaken safety arguments
Not just text: the visual toolkit
Common-Cause vs Cascading Animator
Animated contrast showing one shared root striking both elements at once versus a failure propagating element to element.
Coupling-Factor Radial Map
Radial view of coupling-factor categories around a pair of elements, exposing every shared resource and environment that could host a DFI.
DFA Lifecycle Timeline
V-model timeline placing DFA iterations at concept, system, hardware, and software phases with their inputs and outputs.
Redundancy vs Independence Comparator
Side-by-side comparison of redundant channels that share elements against channels with verified independence, showing how coupling erodes the benefit.
Freedom From Interference Partitioning
Memory, timing, and exchange partitioning between mixed-ASIL software components, highlighting where interference paths must be blocked.
Beta-Factor Sensitivity Curve
Sensitivity plot showing how the common-cause beta factor degrades the effective failure rate of a redundant pair as coupling rises.
Dual-Channel Brake-by-Wire DFA
A full DFA pass over a redundant pressure-sensor pair and a primary plus secondary ECU, working from the stated independence claims down to a closed DFI register.
- Map the architecture and list every independence and FFI claim it relies on
- Hunt DFIs across shared 5 V supply, shared CAN segment, and shared compartment
- Score each candidate by coupling strength and consequence severity
- Assign mitigations such as independent supplies, end-to-end protection, and physical separation
- Track open items and define the evidence needed to close each one
Unlock the full register and mitigation set
Who this guide is for
- Engineers relying on ASIL decomposition who must defend the independence requirement
- Hardware designers with lockstep cores or redundant sensor channels
- Software architects hosting mixed-ASIL components on one microcontroller
- Reviewers and assessors checking whether an independence argument actually holds
Frequently Asked Questions
Common questions about Dependent Failure Analysis (DFA)
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