Obtaining mental workload levels during task perfor- mance is a difficult procedure. The workload level expe- rienced by an operator can affect task performance. This effect can be caused by either excessive or reduced mental workload. Thus, estimating workload levels can help iso- late sources that affect performance. Several techniques can be used to measure operator workload. Psychophysi- ological measurements using electroencephalograms (EEGs) have been used with much success. However, EEG machines are not the most portable, practical, or ubiquitous devices, which constrains their usefulness in experiments. Obtaining physiological mental workload data is a nontrivial task, and there has been a large amount of work on designing lightweight and portable workload measuring devices (Kramer, 1991). Workload assessment techniques should possess the following properties: sensitivity, diagnostic capabilities, selectivity, low intrusiveness, reliability, and ease of im- plementation (Eggemeier, 1988). The NASA Task Load Index (TLX) has been shown to meet these criteria (Rubio, Díaz, Martín, & Puente, 2004). It is a subjective workload assessment technique that relies on a multidimensional construct to derive an overall workload score based on a weighted average of ratings on six subscales: mental de- mand, physical demand, temporal demand, performance, effort, and frustration level. The use of these six subscales to compute an overall workload score has been found to reduce variability among subjects, relative to a unidi- mensional workload rating, while providing diagnostic information about workload sources (Hart & Staveland, 1988). Specific sources of mental workload imposed by different tasks are an important determinant of workload experiences. Three of the subscales relate to the demands imposed on the subject (mental, physical, and temporal demand), whereas the other subscales focus on the inter- action of the subject with the task (performance, effort, and frustration level). Descriptions of the six subscales are shown in the Appendix. Thus, the NASA TLX combines subscale ratings that are weighted according to their sub- jective importance to subjects for a specific task. The NASA TLX has been tested in experimental tasks (e.g., the Fittsberg task, choice reaction time, same/ different judgments, mental arithmetic, mental rotation, etc.), task supervisory control simulations, and flight simulators. The results of the first validation studies are summarized in Hart and Staveland (1988). The NASA TLX has been used in a variety of fields (Hart, 2006). It has been used in studies involving the evaluation of visual and/or auditory displays, vocal and/or manual input devices, and virtual/augmented vision. Stud- ies have explored the relationship between NASA TLX ratings and other performance factors, such as fatigue (Baulk et al., 2007), stress (Reilly, Grasha, Matthews, & Schafer, 2003), trust (Turner, Safar, & Ramaswamy, 2006), experience (Greenwood-Ericksen, Oron-Gilad, Szalma, Stafford, & Hancock, 2006), and situational awareness (Kaber, Onal, & Endsley, 2000). Other NASA TLX stud- ies have involved measures of physiological (e.g., cardio- vascular, muscular, and skin- or brain-related) function thought to index different aspects of workload (Miyake, 2001; Ryu & Myung, 2005). Other studies have used modified versions of the origi- nal NASA TLX. The use of an unweighted or raw TLX (RTLX) is the most common, because high correlations have been shown between the weighted and unweighted scores (Byers, Bittner, & Hill, 1989; Moroney, Biers, 113 © 2009 The Psychonomic Society, Inc. NASA TLX: Software for assessing subjective mental workload ALEX CAO, KESHAV K. CHINTAMANI, ABHILASH K. PANDYA, AND R. DARIN ELLIS Wayne State University, Detroit, Michigan The NASA Task Load Index (TLX) is a popular technique for measuring subjective mental workload. It relies on a multidimensional construct to derive an overall workload score based on a weighted average of ratings on six subscales: mental demand, physical demand, temporal demand, performance, effort, and frustration level. A program for implementing a computerized version of the NASA TLX is described. The software version as- sists in simplifying collection, postprocessing, and storage of raw data. The program collects raw data from the subject and calculates the weighted (or unweighted) workload score, which is output to a text file. The program can also be tailored to a specific experiment using a simple input text file, if desired. The program was designed in Visual Studio 2005 and is capable of running on a Pocket PC with Windows CE or on a PC with Windows 2000 or higher. The NASA TLX program is available for free download. Behavior Research Methods 2009, 41 (1), 113-117 doi:10.3758/BRM.41.1.113 B418 – NT – MJA/cla A. Cao, av9852@wayne.edu