The short answer
Core relationship: An OR gate means any child event can produce the parent. An AND gate means all listed children must coexist. The choice should describe the mechanism, not the diagram shape that looks convenient.
For fault tree analysis with and and or conditions, begin with define the top event and finish with stop at actionable basics. The sequence keeps the starting facts, intermediate result, and final limit visible instead of relying on memory.
A reliable method
Define the top event
State the unwanted condition with system boundary and time. Capture the result before moving to ask how it can occur. A written output shows whether the next deduction follows from the evidence or quietly adds a new assumption.
Ask how it can occur
Create immediate causal events, not distant background factors. Compare this output with the warning about mixing sequence with combination. If that failure still fits, revise the representation before using the result in another deduction.
Choose gate logic
Test whether one child is enough or all are required. Use choose gate logic as a checkpoint. Name the input, the change made, and the conclusion that becomes available so another reader can reproduce the move.
Stop at actionable basics
End branches where events can be measured, controlled, or assigned data. Finish by stating what this step establishes and what it leaves undecided. That boundary prevents a valid local conclusion from being stretched beyond the question.
Worked case
Define the top event in the case
Service unavailability may occur if the primary system fails OR connectivity fails. A data loss branch may require backup failure AND an unrecoverable primary failure, making the combination more specific.
Why stop at actionable basics matters
The case starts with define the top event, then uses ask how it can occur to control the next inference. The conclusion is acceptable only after stop at actionable basics. Changing any fact that supports those moves can change the answer, while an unrelated detail should not.
Failure modes to watch
Mixing sequence with combination
A gate shows logical combination unless time order is explicitly modeled. Return to define the top event and rebuild only from information the problem actually provides.
Dependencies ignored
Shared causes can make branches correlated. Compare the disputed move with choose gate logic; the two should use the same definitions and direction.
Infinite decomposition
More detail is not useful once events cannot be observed or acted upon. Run the practice task while deliberately watching for infinite decomposition. Record the first point where the reasoning begins to drift.
Check against mixing sequence with combination
Questions for ask how it can occur
- Can another reader reproduce define the top event from the stated information?
- Did you complete ask how it can occur without reversing or enlarging the claim?
- Does the result survive the failure mode mixing sequence with combination?
- Which part remains uncertain after stop at actionable basics?
This check is specific to the method above. A clear answer should expose the evidence chain and the point where a different fact would produce a different conclusion.
Try the method
Draw a small fault tree for a missed notification. Include one OR branch and one AND combination, then justify each gate in words.
Continue after stop at actionable basics
Use the root cause analysis guide for the broader method, then apply the idea in the root cause map. Keep the written reasoning so the next attempt can improve a specific step rather than simply repeat the exercise.
Compare this method with Create a Fishbone Diagram That Leads to Evidence, then continue to Process Mapping to Find Delays and Handoff Failures for a different reasoning task.