How can a root cause analysis mislead when only one proposed cause is investigated?

Why

When a team suspects a cause early, such as a new raw material lot, the investigation tends to look for data about that lot. Some data will usually fit, because many things change at once in a plant. The result is a story that is consistent but not discriminating: it would look the same if the true cause were the new operator or the recalibrated mixer.

ICH Q9(R1) warns that subjectivity enters risk work through bias and poorly defined questions and must be managed with data. Patient safety research describes root cause analysis as prone to hindsight bias and to simple linear narratives that ignore multiple interacting contributions. The guard is to list plausible causes before collecting evidence, then for each ask what you would expect to see if it were true and if it were false.

If the new lot is the cause, earlier lots should be fine and the problem should follow the lot to another line. If it does not, the hypothesis weakens. Ruling causes out is as important as ruling one in, and it should be written in the report.

Inspectors read an investigation that considered one cause as an investigation that stopped early. The limit is resources. Not every cause can be tested, so risk and plausibility decide which get the effort, and the report should say which remained untested.