This paper addresses four issues in the design and execution of behavioral observation in classrooms. These four issues relate to the consequences of using different observation intervals, schedules of observation, student sampling methods, and definitions of on-task and off-task behavior for reliability, means, and correlations of time on-task and achievement. A field study observed 108 students in 18 elementary classrooms. Pre and post-achievement data were also collected. The data permit simulations of different intervals, schedules, sampling methods, and definitions for determination of their effects on the outcomes of behavioral observation. Findings suggested that: (1) altering definitions of time-on-task to include momentary off-task behaviors affected the conclusions for the importance of time-on-task; (2) sampling segments of instruction would tend to obscure the positive results for time-on-task; (3) reducing the number of days of observation also weakened the effects of time-on-task; (4) timing of the observation was not very important for the noted effects; and (5) reducing the number of students to less than six may adversely affect reliability. (Author/RL)