The NASA Task Load Index (NASA-TLX) was developed to measure the overall complexity of a task in outer space. This allows different tasks to be compared for their overall complexity for a human to complete. It can be used in more down to earth applications such as healthcare.
Any task can be evaluated in terms of 6 task scales (Figure 8, page 169):
(1) mental demand (MD)
(2) physical demand (PD)
(3) temporal demand (TD)
(4) own performance (OP) based on satisfaction with performance
(5) effort (EF) based on mental and physical demands
(6) frustration level (FL)
Parameters:
(1) tally of relative weights for the 6 task scales
(2) rating for each task dimension
The tally of relative weights involves comparing the 15 possible task pairings and determining which of the 2 is a greater source of workload variation when performing the task. This is based on paired combinations, or (6 * 5) / 2 = 15 combinations with each task scale listed 5 times.
Task scale pairs:
(1) PD vs MD
(2) TD vs MD
(3) OP vs MD
(4) FR vs MD
(5) EF vs MD
(6) TD vs PD
(7) OP vs PD
(8) FR vs PD
(9) EF vs PD
(10) TD vs OP
(11) TD vs FR
(12) TD vs EF
(13) OP vs FR
(14) OP vs EF
(15) EF vs FR
The weight for each task scale is the number of pairs for which it was considered more important. Thus each task scale may appear from 0 to 5 times.
total sum of weights =
= SUM(weights for all 5 task scales) = 15
Each rating scale in performing the task is then rated using a visual analogue scale (VAS) scored from 0 to 100 with ends anchored as:
(1) mental demand (MD) from low to high
(2) physical demand (PD) from low to high
(3) temporal demand (TD) from low to high
(4) own performance (OP) from excellent to poor
(5) effort (EF) from low to high
(6) frustration level (FL) from low to high
For each rating scale the product of each weighting and rating is then calculated.
product for each rating scale =
= (weight from 0 to 5) * (rating from 0 to 100)
overall sum for the rating scales =
= SUM(products for each rating scale)
mean weighted workload (WWL) score =
= (overall sum for rating scales) / (total sum of weights) =
= (overall sum for rating scales) / 15
Interpretation:
• minimum overall sum of rating scales: 0
• maximum overall sum of rating scales: 1,500
• minimum mean weighted workload score: 0
• maximum mean weighted workload score: 100
• The higher the mean weighted workload score the greater the overall demand.
• A task with a greater mean weighted workload score is more demanding than one with a lower mean weighted workload score.
