When performing daily activities, people use internally generated cognitive strategies, or ways of thinking, to plan and execute appropriate actions. This is known as applied cognition. This article describes a study that sought to measure the efficiency of applied cognition of 190 persons with non-dementing Parkinson's disease who were living at home. Participants were asked to perform a usual activity in their own homes. Their performance was videotaped and scored by assessors. The standardized observation instrument measured the effective use of 34 applied cognitive strategy behaviours. These behaviours represent 4 information-processing domains: attention and sensory processing (Perceive), accessing task-related knowledge (Recall), planning and evaluation (Plan), and performance control (Perform). The results indicated that applied cognitive strategies used to plan, evaluate and control performance were the least efficient. The most efficient were those used for sensory discrimination and recalling facts. The subgroup with the more advanced Parkinson's disease had the least efficient use of applied cognition. The results may inform further development of suitable interventions to optimize activity performance. Objective: To explore the use of applied cognitive strategy behaviours during performance of daily activities in people with Parkinson's disease. Design: Quantitative cross-sectional design. Methods: A total of 190 persons living at home with non-dementing Parkinson's disease were videotaped while performing a self-chosen activity in their natural environment. The videotaped performance was scored using the "Perceive, Recall, Plan & Perform System of Task Analysis" to measure: (i) performance mastery; and (ii) effective use of 34 cognitive strategy behaviours covering: attention and sensory processing (Perceive), accessing taskrelated knowledge (Recall), response planning and evaluation (Plan) and performance control (Perform). Mean performance mastery and a hierarchy of least to most effective applied cognitive strategy behaviours were determined for the total group and for 2 subgroups based on disease severity. A multifaceted Rasch model was used for data analysis. Results: Mean performance mastery was 56% (standard deviation 28%). Least efficient cognitive strategy behaviours were those used for planning, evaluating and controlling performance and most efficient strategies were those used for sensory discrimination and recalling factual information. More advanced disease indicated less efficient use of applied cognition. Conclusion: The results suggest that the efficiency of applied cognitive strategy behaviours is compromised in a certain pattern in people with Parkinson's disease, and that it declines with disease progression.