Information Technology Reference
In-Depth Information
•
Don'tlettheaxisforthecontinuousvari-
able go any higher than the maximum value
that's theoretically possible. For example, if
you're plotting percentages of users who
completed each task successfully, the the-
oretical maximum is 100%. If some val-
ues are close to that maximum, Excel and
other packages will tend to automatically
increase the scale beyond the maximum,
especially if error bars are shown.
Mean Times (sec) Across Trials
Design 1
Design 2
Design 3
30
25
20
15
10
5
2.7.2 Line Graphs
Line graphs (
Figure 2.11
) are used most com-
monly to show trends in continuous variables,
often over time. Although not as common as bar
graphs in presenting usability data, they certainly
have their place. The following are some of the
key principles for using line graphs.
0
Trial 1
Trial 2
Trial 3
Trial 4
Mean Times Across Trials
30
25
20
•
Linegraphsareappropriatewhenyouwant
to present the values of one continuous
variable (e.g., percent correct, number of
errors) as a function of another continuous
variable (e.g., age, trial). If one of the vari-
ables is discrete (e.g., gender, participant,
task), then a bar graph is more appropriate.
15
10
5
0
•
Show your data points. Your actual data
points are the things that really matter, not
the lines. The lines are just there to con-
nect the data points and make the trends
more obvious. You may need to increase
the default size of the data points in Excel.
Trial 1
Trial 2
Trial 3
Trial 4
Figure 2.11 Good (top) and bad (bottom) examples of line
graphs for the same data. Mistakes in the bad version include
failing to label the vertical axis, not showing data points, not
including a legend, and not showing confidence intervals.
•
Uselinesthathavesufficientweighttobeclear.Verythinlinesarenot
only hard to see, but it's harder to detect their color and they may imply
a greater precision in data than is appropriate. You may need to increase
the default weight of lines in Excel.
•
Includealegendifyouhavemorethanoneline.Insomecases,itmaybe
clearer to move the labels manually from the legend into the body of the
graph and put each label beside its appropriate line. It may be necessary
to do this in PowerPoint or some other drawing program.
•
Aswithbargraphs,theverticalaxisnormallystartsat0,butit'snotasimpor-
tant with a line graph to always do that. There are no bars whose length is
important, so sometimes it may be appropriate to start the vertical axis at a
higher value. In that case, you should mark the vertical axis appropriately.
The traditional way of doing this is with a “discontinuity” marker ( ) on that
axis. Again, it may be necessary to do that in a drawing program.
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