Interval recording procedures are used by persons who collect data through observation to estimate the cumulative occurrence and nonoccurrence of behavior/events. Although interval recording procedures can increase the efficiency of observational data collection, they can also induce error from the observer. In the present study, 50 observers were trained to discriminate between safe and at-risk behavior, and were then exposed to two types of interval recording procedures: momentary time sampling (MTS) and whole-interval recording (WIR). The extent and sources of observer error were measured with signal detection methods ("d'" and "c"). The results show that observers had greater misclassification of behavior when using WIR, over MTS. The findings also revealed that observers were consistently biased toward classifying behavior as "safe" when using WIR; safe bias was also found with use of MTS, but only after previous use of WIR. (Contains 1 figure.)