An MD&DI January 1997 Feature Article
Task analysis is a highly effective tool for studying the way people
think about and use a product. This tool has its roots in the early time-motion
studies of Gilbreth and Taylor. F. B. Gilbreth, a contractor, observed that no
two bricklayers used the same technique to lay a brick. Using a time-motion
analysis technique, he was able to identify inefficiencies and therefore reduce
the number of discrete motions used to lay a brick--on average from 18 to 4.25.
Today the technique remains the same, although the tools are more sophisticated.
Task analysis can be used to help redesign medical devices such as a
ventilator system for a neonatal intensive-care unit (see figure below). This
system consists of a ventilator, humidifier, oxygen analyzer, alarm unit, and
patient interface device. An optimal design enables a user to operate these
components with the fewest movements.
Figure 1. Depictions of the primary system functions of a ventilator and a map of the device's 24 links.
LEVELS OF ANALYSIS
The task analysis tool has three levels: general task analysis, motion
analysis, and link analysis. Each successive level builds on the information
generated in the previous one.
General Task Analysis. General task analysis is a macrolevel
inventory of the performed tasks. At this level, the designer defines all tasks
and maps their sequence and interactions. On a spreadsheet, the behaviors,
responses, and observations can be scored based on their frequency. This step
can help researchers identify inefficiencies and problems, such as two
successive tasks being too far apart.
Motion Analysis. The second level evaluates the user's body posture
and motion paths. The figure below depicts a map of the physical paths of a
user's hands to illustrate the movement and frequency of each task and its
interdependence with the others. The six-pointed bold-line overlay on the left
indicates six subtasks that compose a primary task. Because all users do not
move in the same way, it is best to test different users to identify variations.
Researchers typically begin with about 25 subjects and add more as needed.
Link Analysis. Link analysis graphically depicts relationships
between tasks. It identifies inefficient sequences and repetitive motions. This
analysis links tasks together in order of importance based on their duration,
frequency, and sequence. Use of this analysis on the tasks in the figure
identifies 24 links and illustrates inefficient placement of controls. For
example, nodes 6, 7, 12, 11, 5, 8, 9, 22, 21, and 16 are clustered, indicating
that a redesigned system should organize and consolidate these controls into a
single functional ar- ray. Link analysis can also be conducted on specific
subtasks. For example, the ventilator has two modes of operation. Plots of each
mode could illustrate a logical left-to-right and top-to-bottom layout of
controls based on their impor- tance and frequency.
The task analysis technique is a powerful tool that can be used to
integrate into the design process how people use a product. This technique
provides the following information about the product studied:
- The priority of all tasks being performed.
- The configuration of the six main interface nodes.
- The configuration for primary control elements.
- The configuration for ventilator mode controls.
- The amount of time it takes to execute both subtasks and the primary task.
- A measure of the current interface to which a new design can be compared.
Through use of these results in the design process, a product can be
developed that incorporates the most efficient path for users. The resulting
redesigned device should be easier to use and better organized than the