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Chapter 8: Making Software Performance Measurements
Duration Performance Measurements
Standard Deviation Deviation from the mean of time. The following equation
is used to calculate standard deviation:
Where:
N
mean
S
Error Tolerance and Confidence Level An approximate error may exist in
displayed information. Error tolerance for a level of confidence is calculated using
the mean of the standard deviations and the mean of the means. Error tolerance
gives an indication of the stability of the information. For example, if the error is
5% for a confidence level of 95%, then you can be 95% confident that the
information has an error of 5% or less.
The Student's "T" distribution is used in these calculations because it improves the
accuracy for small samples. As the size of the sample increases, the Student's "T"
distribution approaches the normal distribution.
The following equation is used to calculate error tolerance:
Where:
O
t
N
252
Number of intervals.
Average time.
Sum of squares of time in the intervals.
sumq
Mean of the standard deviations in each time range.
m
Table entry in Student's "T" table for a given confidence
level.
Number of intervals.
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