Background - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 32

The actual range is the number of people successful on the task for each group. The
expected range is the total number of people successful (33) divided by the number of
groups (3), or 11 in this example. The expected value is what you would expect if there were
no differences among any of the three groups.

Figure 2.8 shows what the data look like and output
from the CHITEST function. In this example, the like-
lihood that this distribution is due to chance is about
2.9% (0.029). Because this number is less than 0.05
(95% confidence), we can reasonably say that there is a
difference in success rates among the three groups.

In this example we were just examining the distri-
bution of success rates across a single variable (experi-
ence group). There are some situations in which you
might want to examine more than one variable, such
as experience group and design prototype. Performing
this type of evaluation works the same way. Figure 2.9
shows data based on two different variables: group
and design. For a more detailed example of using χ 2
to test for differences in live website data for two alter-
nativepages(so-calledA/Btests),seeChapter 9.

Figure 2.8 Output from a χ 2 test in Excel.

2.7 PRESENTING YOUR DATA
GRAPHICALLY
You might have collected and analyzed the best set of
usability data ever, but it's of little value if you can't
communicate it effectively to others. Data tables are
certainly useful in some situations, but in most cases
you'll want to present your data graphically. A num-
ber of excellent topics on the design of effective data
graphs are available, including those written by Edward Tufte (1990, 1997, 2001,
2006), Stephen Few (2006, 2009, 2012), and Dona Wong (2010). Our intent in
this section is simply to introduce some of the most important principles in the
design of data graphs, particularly as they relate to user experience data.

Figure 2.9 Output from a χ 2 test with two variables.

We've organized this section around tips and techniques for five basic types
of data graphs:

Column or bar graphs
Line graphs
Scatterplots
Pie or donut charts
Stacked bar or column graphs

We begin each of the following sections with one good example and one bad
example of that particular type of data graph.