The ability of therapists and educators to reliably analyze data from single-subject designs using visual inspection of graphed data was investigated. Sixty-one examiners provided a rating of whether a significant change in performance occurred across the baseline and treatment phases of six graphs of hypothetical data. Results revealed considerable disagreement among the raters on four of them. Further analysis suggested that the visual characteristics of a change in variability or change in slope across the baseline and treatment phases were associated with the most disagreement. The advantages and limitations of visual analysis of graphed data were discussed and the argument made that additional information is needed on the process of visual inspection to ensure reliable interpretation of single-subject data.