MetricS 121
(i) can you explain dre?
DRE (Defect Removal Efficiency) is a powerful metric used to measure
test effectiveness. From this metric we come to know how many bugs we
found from the set of bugs which we could have found. The following is the
formula for calculating DRE. We need two inputs for calculating this metric:
the number of bugs found during development and the number of defects
detected at the end user.
Figure 117 DRE formula
But the success of DRE depends on several factors. The following are
some of them:
Severity and distribution of bugs must be taken into account.
Second, how do we confirm when the customer has found all
the bugs. This is normally achieved by looking at the history of
the customer.
(B) can you explain unit and system test dre?
DRE is also useful to measure the effectiveness of a particular test such as
acceptance, unit, or system testing. The following figure shows defect numbers
at various software cycle levels. The 1 indicates that defects are input at
the phase and2indicates that these many defects were removed from that
n
n
122 Software teSting interview QueStionS
Figure 118 Defect injected and removed per phase
particular phase. For instance, in the requirement phase 100 defects were
present, but 20 defects are removed from the requirement phase due to a
code review. So if 20 defects are removed then 80 defects get carried to the
new phase (design) and so on.
First, let’s calculate simple DRE of the above diagram. DRE will be the
total bugs found in testing divided by the total bugs found in testing plus the
total bugs found by the user, that is, during acceptance testing. So the following
diagram gives the DRE for the those values.
MetricS 123
Now let’s calculate system DRE of the above given project. In order
to calculate the system DRE we need to take the number of defects found
during the system divided by the defects found during system testing plus
the defects found during acceptance testing. The following figure shows the
system DRE calculation step by step.
Figure 119 DRE calculation
Figure 120 System testing DRE calculation
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