Inspect a failure-condition tree without inventing probabilities

A diagram of possible failure conditions can be useful without assigning probabilities or claiming to explain an actual incident. Its logic must still be clear. Work through a deliberately small fictional publishing model with colleagues, using an existing document to preserve definitions and assumptions. The output is an inspectable model, not a forecast or a diagnosis of any live system.

Define the unwanted outcome and the leaves

For this fictional practice exercise, the top outcome is publishing the wrong version of a guide. Define A as wrong source selected before review. Define B as the review having no effective check for a wrong source. Define C as the approved source being replaced with a wrong version after review.

Choose the toy model F = (A AND B) OR C. Under this model, a wrong pre-review selection produces the top outcome only when review also fails to detect it. A post-review replacement produces the top outcome through a separate route. These are stipulated exercise rules. They omit other possible publishing failures and do not establish how any actual team operates.

Trace the truth combinations literally

With A, B, and C all false, F is false. A alone leaves F false because B is false. B alone also leaves F false because A is false. A and B together, with C false, make F true.

When C is true, F is true regardless of A and B. That accounts for the four combinations where C is true, including C alone. Across the eight possible combinations, the three false outcomes are 000, 100, and 010 when the digits mean A, B, C. The true outcomes are 110, 001, 101, 011, and 111. Keep the digit order alongside the register to avoid mistaking a label change for a different logical result.

Identify minimal sufficient sets within the model

The minimal sufficient sets are {A,B} and {C}. Removing A or B from the first set makes that route insufficient when C is absent. Removing C from the second leaves no condition. Larger true sets, such as {A,B,C}, contain a smaller sufficient set and therefore are not minimal.

Minimal here describes the stated logic, not the most common or most serious real cause. Do not rank these sets by frequency without evidence. The model also does not say A and B are independent. Their meanings may share a source of error, such as both stages relying on the same mislabeled document.

Check what an ambiguous gate would change

Replace the first AND with OR as a deliberate faulty alternative: (A OR B) OR C. Now A alone or B alone produces F, contradicting the original exercise rules. A line or arrow labeled contributes to would conceal this distinction.

Ask reviewers whether the intended relationship really requires both conditions, either condition, or another condition entirely. Keep disputed meanings unresolved until the model owner clarifies them. A neat drawing cannot compensate for leaves that describe different events to different readers. Record any shared assumption that undermines the idea of two separate checks.

Separate model findings from empirical claims

Retain the leaf definitions, literal expression, truth register, minimal sets, and omitted conditions together. Name the observations needed before applying this account to a real process: the actual source selection, review behavior, and whether a post-review replacement was possible.

Do not infer a failure rate, control effectiveness, independence, or complete coverage from the toy tree. Ask another person to explain why A alone is insufficient under the original expression. If their answer depends on unstated assumptions, refine the model. Its useful result is clear reasoning about specified combinations, with real-world uncertainty still visible.