NASA Task Load Index

Acronym: 
NASA TLX
Description: 

The NASA task load index (NASA TLX) is a tool for measuring and conducting a subjective mental workload (MWL) assessment. It allows you to determine the MWL of a participant while they are performing a task. It rates performance across six dimensions to determine an overall workload rating. The six dimensions are as follows:

1. Mental demand – how much thinking, deciding, or calculating was required to perform the task.
2. Physical demand – the amount and intensity of physical activity required to complete the task.
3. Temporal demand – the amount of time pressure involved in completing the task.
4. Effort – how hard does the participant have to work to maintain their level of performance?
5. Performance – the level of success in completing the task.
6. Frustration level – how insecure, discouraged, or secure or content the participant felt during the task.

Each subscale is presented to the participants either during or after the experimental trial. They are asked to rate their score on an interval scale ranging from low (1) to high (20). The TLX also employs a paired comparisons procedure. This involves presenting 15 pairwise combinations to the participants and asking them to select the scale from each pair that has the most effect on the workload during the task under analysis. This procedure accounts for two potential sources of between-rater variability: differences in workload definition between the raters and differences in the sources or workload between the tasks.

The subscales are given to the participants either during or following their experimental trial. The participants self-rate on a scale of 1 (low) to 20 (high) using 15 pairwise combinations designed to elicit from the participants the pair that has the greatest effect on workload while performing the task.

Uses: 

To evaluate mental workload demand on an individual performing a specific task.

How do I use this tool?: 

1. DEFINE THE TASK(S).

2. (Optional) CONDUCT A HIERARCHICAL TASK ANALYSIS (HTA) FOR THE TASK(S) UNDER ANALYSIS.

3. SELECT PARTICIPANTS based on the goals of the analysis.

4. BRIEF PARTICIPANTS by explaining the purpose of the study and the basics of the NASA-TLX method. A workshop on mental workload and a brief runthrough of the NASA-TLX may be useful.

5. PERFORM TASK UNDER ANALYSIS. The participants should perform the tasks and fill out the NASA-TLX form either during the trial or immediately posttrial.

6. FOLLOW WEIGHTING PROCEDURE. Use the WEIGHT software to present the 15 pairwise comparisons to the particpants, asking them to select from each of the 15 pairs the subscale from each pair that contributed the most to the workload of the task.

7. COMPLETE NASA-TLX RATING. Ask participants to give a rating for each subscale from 1 (low) to 20 (high).

8. TLX SCORE CALCULATION. The TLX software can calculate the overall worklaod score between 0 and 100.

Expertise Required: 
Users of this tool usually have some training or experience in its use.
Recommended Supplies/Materials: 
Computer and Internet access
Advantages: 

Provides a quick and simple estimate of operator mental workload.

Generic subscales allow the technique to be used across multiple domains.

Software is available free online.

TLX software eases the burden on the analyst.

Disadvantages: 

Laborious and time consuming to conduct the subscale weighting.

May be intrusive to the task when administered during the trial.

Participants may be prone to correlate their task performance with their workload ratings.

When administered post-task, the participants may forget details of the task.

Where can I go to learn more?: 

NASA. NASA TLX: task load index. 2006 [cited 2010 March 4]; Available from: http://humansystems.arc.nasa.gov/groups/TLX/

Stanton N, Salmon P, Walker G, et al. Mental workload assessment method. Human factors methods: a practical guide for engineering and design. Great Britain: Ashgate; 2005. p. 301-64.