A computer based training program for promoting understanding of NIOSH evaluation guidelines and performance criteria related to collecting air samples for analyzing airborne toxic substances was described. The program was developed primarily for health professionals, Laboratory researchers and technicians, and field personnel and is available in two versions, one to be run on a Macintosh personal computer and one to be run on a DOS personal computer. The program consists of three main sections: a statistics, experiments, and calculations section. The statistics section graphically illustrates important statistical concepts and allows the user to change some values such as precision, bias, and accuracy and to see how the changes affect results. The basis of the NIOSH accuracy criterion (NAC) and related concepts can be examined in screens accessed through a historical background icon appearing on the menu screen. The historical background screens trace the development of ideas at NIOSH concerning bias, precision, accuracy, the detection limit, and establishment of the NAC value of +/-25%. The experiments section summarizes experiments used to generate data for method evaluations utilizing appropriate graphics and animation to illustrate sampling procedures. Screens describe and illustrate analytical recovery, capacity and breakthrough, sample stability, and field evaluations. The calculations section consists of a series of screens that work like a set of customized spreadsheets enabling the user to enter test data for a specific method and to calculate appropriate evaluation information such as precision, bias, accuracy, and detection limit. The screens also enable the user to readily experiment with different values to evaluate their effect on method accuracy, detection limit, or sample stability.