<p>Many aspects of noticing in the classroom are tacitly understood by experienced educators. In this case study, we pair ecological and cognitive psychology with computer-augmented analysis of video data to try to better understand some of the tacit elements of educator noticing and decision-making. The ecological model of noticing views noticing as an ongoing process of looking and listening that enables educators to maintain a grip on unfolding lessons. We collected data from a teacher education class that taught adult students how to teach fractions to children. The study trials the use of computer-assisted analysis techniques to enhance ecological analysis. Analysis of video and post-lesson interview data identified critical events where the educator made decisions mid-lesson. While the educator recalled a range of cognitive processes during these events, she also recalled attending to specific environmental cues. Using an ecological framework, we analysed these cues and used object detection algorithms to identify variation in their structure. This enabled us to identify shifts in student body position and variations in the way that students constructed number lines that the educator used to maintain grip and guide decision-making. Our findings illustrate how an ecological model of noticing gives weight to the analysis of tacit perceptual interaction and variation in environmental entities that an educator attends to guide action mid-lesson.</p>