5 Whys root cause analysis: steps and an example

Why it matters

Correcting a part that sits at an angle in the cap-fitting job keeps production moving; it does not explain why the same defect recurs. The 5 Whys analysis starts from the observed result and investigates the conditions that produced it with successive questions. Each answer is the starting point of the next question. However, a neatly built chain of sentences does not show that the physical process really works that way. A worn sleeve does not prove that the angled fit comes only from wear. The size of the part, the way it is placed or the adjustment condition can produce the same result. The investigation must separate these possibilities without presenting the plausible-looking explanation as a verified cause.

The fishbone diagram helps to see together the alternatives that could lead to the same result. The branches can be set up under headings such as equipment, material, work arrangement, measurement and environment; the headings are not a mandatory checklist. A branch being full does not mean there is a cause in that area. The diagram opens up the investigation area; the cause chain deepens the links of a selected explanation. It is not necessary to draw every branch at the same length or to end every chain with the same number of questions. Priority lies with assumptions that could explain the problem and can be tested with evidence. Density on the page does not measure the maturity of the investigation.

The relation of the evidence to the explanation is assessed separately. The absence of a sleeve measurement in a maintenance schedule can support that the check is not defined. To say that it caused the angled fit, the link between wear, centering and the defect must also be examined. The absence of a record does not always show that the work was not done; the deficiency of the record system is investigated separately. A supported assumption is an explanation consistent with the findings at hand. For a contradicted assumption there is reliable evidence in the opposite direction. For an open assumption there is not enough information to decide. These states can change as the investigation progresses; the reason for the change and the new evidence are recorded.

Ending the investigation at the sentence "K01 was careless" leaves the arrangement that made the error possible invisible. If the same person gives different results under different conditions, this difference must be explained. If a lack of knowledge is claimed, it is shown which job knowledge was not known and how this is tied to the observed defect. Training can be a ready solution; choosing it does not prove a training deficiency. A safe correction can be made without delay, but the temporary protection is kept separate from verifying the cause. The aim of the investigation is not to find the person at fault but to set up a testable working condition that will prevent recurrence. The figure and the short quiz scenarios in this lesson also fall within the synthetic data of the examples. This dataset was generated synthetically for training; it does not represent a real business, employee or customer study.

Formula