Is the reducing effect of the Staff Observation Aggression Scale owing to a learning effect? An explorative study Abstract This study explores the decline in the amount of reported aggression incidents when using the Staff Observation Aggression Scale (SOAS). Because this decline could hold significant implications for practice or research, the objective of this study was to explore the plausible explanations for this phenomenon. The influence of systematic feedback of the gained data was also studied. The SOAS was introduced on a closed ward in a psychiatric institution in the Netherlands as a measuring instrument to assess the amount of aggression incidents during a 16-week period. The standard procedure for reporting general incidents was continued. After 10 weeks of registration, systematic feedback of the gathered data took place. Statistic analysis was used to determine significant changes in the total amount as well as on the various forms of aggression. During this observation period, qualitative data were collected to support the analysis of the quantitative data. A significant decrease in the total amount of reported aggression incidents as well as in the categories of 'verbal aggression' and 'remaining aggression' was found. However, the category 'physical aggression' was stable during the entire observation period. The number of standard procedure reports did not decrease when compared with the pre-test period. Nursing staff did not recognize a reduction in the amount of aggression incidents. The role of a learning effect in the reduction of reports on aggression incidents is not excluded but there are indications that the reduction is also the result of 'reporting fatigue' and 'shifting attention'.