Information Technology Reference
In-Depth Information
NUMBER OF DECIMAL PLACES TO USE WHEN
REPORTING DATA
One of the most common mistakes many people make is reporting data from a usability
test (mean times, task completion rates, etc.) with more precision than it really deserves.
For example, the mean of the times in Table 2.1 is technically 35.08333333 seconds. Is
that the way you should report the mean? Of course not. That many decimal places may
be mathematically correct, but it's ridiculous from a practical standpoint. Who cares
whether the mean was 35.083 or 35.085 seconds? When you're dealing with tasks that
took about 35 seconds to complete, a few milliseconds or a few hundredths of a second
make no difference whatsoever.
So how many decimal places should you use? There's no universal answer, but some of
the factors to consider are accuracy of the original data, its magnitude, and its variability.
The original data in Table 2.1 appear to be accurate to the nearest second. One rule of
thumb is that the number of significant digits you should use when reporting a statistic,
such as the mean, is no more than one additional significant digit in comparison to the
original data. So in this example, you could report that the mean was 35.1 seconds.
2.3.2 Measures of Variability
Measures of variability reflect how much the data are spread or dispersed across
the range of values. For example, these measures help answer the question, “Do
most users have similar task completion times or is there a wide range of times?”
In most usability studies, variability is caused by individual differences among
your participants. There are three common measures of variability: range, vari-
ance, and standard deviation.
The range is the distance between the minimum and maximum values. For
the data in Table 2.1 , the range is 32, with a minimum time of 21 seconds and
a maximum time of 53 seconds. The range can vary wildly depending on the
metric. For example, in many kinds of rating scales, the range is usually limited
to five or seven, depending on the number of values used in the scales. When
you study completion times, the range is very useful because it will help iden-
tify “outliers” (data points that are at the extreme top and bottom of the range).
Looking at the range is also a good check to make sure that the data are coded
properly. If the range is supposed to be from one to five, and the data include a
seven, you know there is a problem.
EXCEL TIP
The minimum of any set of numbers in Excel can be determined using the “=MIN”
function and the maximum using the “=MAX” function. The range can then be deter-
mined by MAX-MIN. The variance can be calculated using the “=VAR” function and the
standard deviation using the “=STDEV” function.
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