Information Technology Reference
In-Depth Information
For example, consider the data shown in
Figure 8.1
from Chadwick-Dias,
McNulty, and Tullis (2003), which shows
z
scores of performance for two itera-
tions of a prototype. This research studied the effects of age on performance in
using a website. Study 1 was a baseline study. Based on their observations of the
participants in Study 1, especially the problems encountered by the older par-
ticipants, they made changes to the prototype and then conducted Study 2 with
a new group of participants. The
z
scores were equal-weighted combinations of
task time and task completion rate.
It's important to understand that the
z
-score transformations were done using
the
full set
of data from Study 1 and Study 2 combined. They were then plotted
appropriately to indicate from which study each
z
score was derived. The key
finding was that the performance
z
scores for Study 2 were significantly higher
than the performance
z
scores for Study 1; the effect was the same regardless of
age (as reflected by the fact that the two lines are parallel to each other). If the
z
-score transformations had been done
separately
for Study 1 and Study 2, the
results would have been meaningless because the means for Study 1 and Study
2 would both have been forced to 0 by the transformations.
Figure 8.1 Data showing performance
z
scores from two studies of a prototype with participants over a
wide range of ages. The performance
z
score was an equal-weighted combination of task time and task
completion rate. Changes were made to the prototype between Study 1 and Study 2. The performance
z
scores were significantly better in Study 2, regardless of the participant's age. Adapted from Chadwick-
Dias et al. (2003); used with permission.
8.1.4 Using Single Usability Metric
Jeff Sauro and Erika Kindlund (2005) developed a quantitative model for
combining usability metrics into a single usability score. Their focus is on task
completion, task time, error counts per task, and post-task satisfaction rating
Search WWH ::
Custom Search