Nasa TLX


NASA-TLX allows users to perform subjective workload assessments on operator(s) working with various human-machine systems. NASA-TLX is a multi-dimensional rating procedure that derives an overall workload score based on a weighted average of ratings on six sub scales.

These sub scales include Mental Demands, Physical Demands, Temporal Demands, Own Performance, Effort and Frustration. It can be used to assess workload in various human-machine environments such as aircraft cockpits, command, control, and communication (C3) workstations; supervisory and process control environments; simulations and laboratory tests.

The NASA TLX known issues

When people evaluate the workload of a task there is no objective standard (e.g., its i "actual" workload) against which their evaluations can be compared. In addition there are no physical units of measurement that are appropriate for quantifying workload or many of its component attributes. This absence of external validation represents one of the most difficult problems encountered in evaluating a candidate workload assessment technique or the accuracy of a particular rating.

There is no objective workload continuum, the "zero" point and upper limits are unclear, and intervals are often arbitrarily assigned. The problem of a "just noticeable difference" is particularly acute in workload assessment, since rating dimensions are often indirectly related to objective, quantifiable, physical dimensions.

The attributes that contribute to workload experiences vary between tasks and between raters because workload is not uniquely defined. The objective qualities of the task demands; workload ratings also reflect an operator's response to the task. Thus, the workload experiences

of different individual  faced with identical task requirements may be quite different because the relationship between objective changes in a task and the magnitudes of workload ratings is indirect rather than direct. This factor distinguishes workload ratings from many other types of judgements. Furthermore.


If workload is caused by one particularly salient source or by very high levels of one or more factors, then it is likely that other factors will not be considered in formulating a workload judgement. Specific workload-related dimensions might be so imperative, or so imbedded in a particular context, that they contaminate other,less subjectively salient factors. Conversely, less salient factors cannot be evaluated without

also considering those that are more salient.


The weighted combination of factors provides a sensitive indicator of the overall workload between different tasks and among different levels of each task, while the weights and the magnitude of the ratings of the individual scales provide important diagnostic information about the specific source of loading within the task.