From The Ergoweb® Learning Center

A Tool to Quantify Mental Workload

A quantifiable evaluation of workplace mental workload was created by authors Pretorius and Cilliers through a participatory process involving train control center workers. Called the mental workload index (MWLI), the tool measures overall workplace mental workload as opposed to subjective impression of mental workload felt by an individual worker. This style of assessment can assist management in evaluating work volume/nature in order to properly allocate resources to optimize safety.
Although developed for application in railroad industry, a mental workload formula could be developed for nearly any workplace setting by following the authors’ methodology.
Formula Development
A train control center stakeholder group composed of technical experts, experienced radio train order system workers, and labor representatives was formed. Through an analysis of train control officer (TCO) activity and following a consensus process, the stakeholder group identified three key task elements (descriptive of the work content):
  1. Data transactions – information gathered by a TCO beyond that needed for an authorization (i.e., estimated time of train arrival/departure or equipment breakdown).
  2. Authorizations – approval of train movement to a specific location.
  3. Number of telephone and radio communications – relaying a variety of information to facilitate operations (i.e., maintenance requests, breakdown info).
The stakeholder group also identified a set of modifiers that impact mental workload including:
  1. Shift – time period relative to day and time duration of shift.
  2. Experience – number of years performing the job.
  3. Interface complexity – interface with other train control systems.
  4. Running times – limited or great variation in duration of train running over a specific area.
  5. Crossing places – uniform or varying gradients; types of points.
  6. Platform location – whether or not there is a platform; platform design
  7. Number of authorizations vs. number of crossing places – complexity of planning.
  8. Type and mix of trains – train characteristic/priority.
  9. Locomotive depots – presence and number of depots.
  10. Presence of shunting yards/activities – presence of shunting yards.
  11. Topography – terrain/landscape of the train run.
It was recognized that a task element or moderating factor may vary in its contribution to mental workload. This concern was resolved by using a weighting value to the variables. The key task elements and moderating factors along with their weighted values are provided in Tables 1 and 2.
Task Element
Weighted Value
Data transactions
Number of telephone and radio communications
Table 1: Task elements and their weighted value. If 11 Authorizations are given over the course of a shift, the value of the Authorizations Task Element is 33 (11 x 3).
Scale range
Maximum Weight (%)
Interface complexity
Running times
Crossing places
Platform location
Number of authorizations vs. number of crossing places
Type and mix of trains
Locomotive depots
Presence of shunting yards/activities
Table 2: For each modifier, the scale range and the maximum weight that can be contributed. If the Shift of a TOC was from 18:00 to 06:00 hours, the Shift Modifier was given a scale value of 5.0 and an overall weighted value of 1.120.
The equation that reflects the relationships of Task Elements and Modifiers to mental workload is:
MWLI =(sum of weighted task elements) x (product of weighted modifier factors)
Formula Testing
Thirty-six train control centers were ranked from high to low mental workload demand by users of the system. These same 36 centers were assessed for the task elements and modifiers as per MWLI definitions. MWLI scores ranged from 89 to 5789. There was a close correlation between the subjective ranking of a center’s mental workload and the relative MWLI score. The MWLI value indicated the degree of difference between centers due to its quantitative output.
Formula validity and reliability were furthered by meeting accepted criteria such as: 
  • Sensitivity – changes in workload such as task difficulty or resource demand were perceived
  • Diagnosticity – source of the workload was identifiable
  • Selectivity – variables that reflect only mental workload were measured
  • Intrusiveness – the workplace was not disturbed while applying the tool
  • Operator acceptance – significant work factors were captured to obtain confidence from the operator; the formula was developed with user participation
Study Limitations
Reservations effecting of this research include:
  1. The sample size of 36 train control centers was considered small.
  2. Further validity testing is needed.
Article Title: Development of a mental workload index: A systems approach
Publication: Ergonomics 50:9, 1503-1515, 2007
Authors: A Pretorius and P J Cilliers

This article originally appeared in The Ergonomics Report™ on 2007-09-26.