[en] Under normal circumstances, operating decisions are made in accordance with the operators understanding of written procedures and of the plant which they are controlling. To ensure plant safety, an operator must have sufficient knowledge to respond correctly when both routine and unusual events place demands on his knowledge and understanding of the plant and procedures. In order that plant operating procedures or the methods used in qualifying operators may be improved a quantitative measure of understanding is required. This paper proposes a methodology which could be developed to yield a quantitative assessment of an individual's understanding. An extension is outlined where the knowledge relevant at each stage of a progressing task is assessed. This is intended to allow the identification of those parts of a task which are only weakly supported by an operator's background knowledge and where for example additional training and/or improved documentation could be introduced to greatest effect. (author). 12 refs, 4 figs