Background - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 23

• Thealphalevelwewanttoadopt.Themostcommonalphalevels(pri-

marily by convention) are 5 and 10%. Let's choose an alpha of 5% for
this example, which is a 95% confidence interval.

The 95% confidence interval is then calculated using the following formula:

Mean ± 19. * [standarddeviation/sqrt(sample size)]

The value “1.96” is a factor that reflects the 95% confidence level. Other con-
fidence levels have other factors. This formula shows that the confidence interval
will get smaller as the standard deviation (the variability of data) decreases or as
the sample size (number of participants) increases.

EXCEL TIP
You can calculate the confidence interval quickly for any set of data using the
CONFIDENCE function in Excel. The formula is easy to construct:

= CONFIDENCE(alpha, standarddeviation, sample size)

Alpha is your significance level, which is typically 5% (0.05) or 10% (0.10). The stan-
dard deviation can be calculated using the STDEV function. The sample size is simply
the number of cases or data points you are examining, which can be calculated using
the COUNT function. Figure 2.1 shows an example. For the data in Table 2.1 , the result
of this calculation is 6.4 seconds. Since the mean is 35.1 seconds, the 95% confidence
interval for that mean is 35.1±6.4, or 28.7 to 41.5 seconds. So you can be 95% certain
that the true population mean for this task time is between 28.7 and 41.5 seconds.

Figure 2.1 Example of how to calculate a 95% confidence interval using the “confidence” function in
Excel.