Critical process safety decisions are made using the results of PHA studies so worksheet entries must be accurate and defensible. Entries should not be based on undocumented, hidden, unsupported, or invalid assumptions. For example, study teams should not simply assume that relief valve sizing is adequate. There should be confirmation that the adequacy of relief valve sizing has been addressed.

Study team members must be alert to assumptions that are made by the team and challenge them when they do not have a sound foundation. There is little point in performing a high quality study on a process that does not actually exist because invalid assumptions have been made.

Sometimes assumptions are made during the performance of a PHA study to avoid suspending a study session to seek their confirmation, for example, assumptions regarding the accuracy of documents or how the process operates. They should be documented and confirmation obtained promptly to avoid possible study re-work if they prove to be unfounded.