Discontinuous measurement systems are often used to record the occurrence of physical activity. These measures, however, provide only an estimation of the occurrence of a particular behavior. In the current study, several commonly used interval recording methods were utilized to examine the datasets of the physical activity of four preschoolers. The results indicated that discontinuous recording significantly overestimated the occurrence of physical activity, and progressively overestimated activity to a greater degree as the interval length increased. In addition, the degree of overestimation varied as a function of activity level (low, medium, high activity). These results suggest that it is important to evaluate a measurement system before using it to measure a particular behavior, in that nay interpretation of those data may differ depending on the observation system utilized.