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Standard Deviation Deviation from the mean of time. The following
equation is used to calculate standard deviation:
Where:
N
Number of intervals.
mean
Average time.
S
Sum of squares of time in the intervals.
sumq
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
Mean of the standard deviations in each time range.
m
t
Table entry in Student's "T" table for a given confidence level.
N
Number of intervals.
P
Mean of the means (i.e., mean of the average times in each time range).
m
Making Software Performance Measurements
To interpret duration measurement reports
281
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