Background - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 30

It does not necessarily mean that each of the
design means is significantly different from
each of the others—only that there is an effect
overall. To see if any two means are significantly
different from each other, you could do a two
sample t test on just those two sets of values.

Task Rating vs. Task Time

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2.5 RELATIONSHIPS BETWEEN
VARIABLES
Sometimes it's important to know about the
relationship between different variables. We've
seen many cases where someone observing
a usability test for the first time remarks that
what users say and what they do don't always
correspond with each other. Many users will
struggle to complete just a few tasks with a
prototype, but when asked to rate how easy or
difficult it was, they often give it good ratings.
This section provides examples of how to perform analyses that investigate these
kinds of relationships (or lack thereof).

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R 2 = 0.26

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0 1 2 3 4 5 6

Mean Task Time (mins)
Figure 2.7 An example of a scatterplot (with trend line) in Excel.

2.5.1 Correlations
When you first begin examining the relationship between two variables, it's
important to visualize what the data look like. That's easy to do in Excel using a
scatterplot. Figure 2.7 is an example of a scatterplot of actual data from an online
usability study. The horizontal axis shows mean task time in minutes, and the
vertical axis shows mean task rating (1-5, with higher numbers being better).
Note that as the mean task time increases, the average task rating drops. This
is called a negative relationship because as one variable increases (task time),
the other variable decreases (task rating). The line that runs through the data is
called a trend line and is added easily to the chart in Excel by right-clicking on
any one of the data points and selecting “Add Trend Line.” The trend line helps
you better visualize the relationship between the two variables. You can also
have Excel display the R 2 value (a measure of the strength of the relationship) by
right-clicking on the trend line, choosing “Format Trend Line,” and checking the
box next to “Display R-squared value on chart.”

EXCEL TIP
You can calculate the strength of the relationship between any two variables (such as
task time and task rating) using the CORREL function in Excel:

= CORREL(Array 1, Array 2)

Array 1 and Array 2 are the two sets of numbers to be correlated. The result will be a cor-
relation coefficient, or “r”. For the data represented in Figure 2.7 , r = −0.53. A correlation