Four observer groups with different levels of expertise were tested in an investigation into the comparative nature of expert performance. The radiological task was the detection and localisation of significant pulmonary nodules in postero-anterior views of the chest in adults. Three test banks of 40 images were used. The observer groups were 6 experienced radiologists, 6 experienced radiographers prior to a six month training programme in chest image interpretation, the same radiographers after their training programme, and 6 fresher undergraduate radiography students. Eye tracking was carried out on all observers to demonstrate differences in visual activity and nodule detection performance was measured with an AFROC technique. Detection performances of the four groups showed the radiologists and radiographers after training were measurably superior at the task. The eye-tracking parameters saccadic length, number of fixations visual coverage and scrutiny time per film were measured for all subjects and compared. The missed nodules fixated and not fixated were also determined for the radiologist group. Results have shown distinct stylistic differences in the visual scanning strategies between the experienced and inexperienced observers that we believe can be generalised into a description of characteristics of expert versus non-expert performance. The findings will be used in the educational programme of image interpretation for non-radiology practitioners.