Information Technology Reference
In-Depth Information
Participant
P1
X
X
X
X
X
X
0.6
P2
X
X
X
X
0.4
P3
X
X
X
X
X
0.5
P4
X
X
X
X
X
X
0.6
P5
X
X
X
0.3
P6
X
X
X
X
0.4
P7
X
X
X
X
X
X
0.6
P8
X
X
X
X
X
X
X
0.7
P9
X
X
X
X
X
X
0.6
P10
X
X
0.2
Proportion
0.5
0.5
0.5
0.4
0.6
0.4
0.4
0.6
0.6
0.4
0.49
Once the average proportion has been determined (0.49 in this case), the next step is to
calculate how many users are needed to identify a certain percentage of issues. Use the
following formula:
p n
1−−
(
) ,
where n is the number of users.
So if you want to know the proportion of issues that would be identified by a sample of
three users:
1−(1−0.49) 3
1−(0.51) 3
1−0.133
0.867,orabout87%,oftheissueswouldbeidentifiedwithasampleofthreeusers
from this study
5.7.2 Five Participants is Not Enough
Other researchers have challenged this idea of the magic number 5 (Molich
et al.,1998;Spool&Schroeder,2001;Woolrych&Cockton,2001).Spooland
Schroeder (2001) asked participants to purchase various types of products, such
asCDsandDVDs,atthreedifferentelectronicswebsites.Theydiscoveredonly
35%oftheusabilityissuesafterthefirstfiveparticipants—farlowerthanthe
 
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