In managing complex manufacturing operations, what situations require manual intervention, requiring operator skill and experience? How does one differentiate between tampering and adjustment? This work develops a method for including tacit operator decision-making into the plant model. We show how a combined usage of systems engineering, statistics, and quality principles may be used to identify where quality-critical operator judgements were occurring, where these had previously been hidden to the operators and managers. The systems engineering perspective provides a method to develop the plant model, the provision of production insights to inform the interpretation of the statistics, and the representation of the operator judgements. Suitable mechanisms for the statistical analysis were identified as control charts, ANOVA, feature selection, and link analysis. It was shown how these could singly or collectively be used to identify areas in which to prospect for operator judgement activities.