Expert judgment is a method that is based on consultation with one or more experts that have experience in and knowledge of a topic. This technique is generally used when other methods are not feasible and/or additional validation is required. It is used, e.g. for completing, validating, interpreting and integrating existing data; assessing the impact of a change; predicting the occurrence of future events and the consequences of a decision, etc.
It is usually part of a forecasting approach, but lacks validation when it is the sole component.