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4-16 Performance Measurements
Standard Deviation. Deviation from the mean of time. The
following equation is used to calculate standard deviation:
Where:
Number of intervals.
N
Average time.
mean
Sum of squares of time in the intervals.
S
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:
Mean of the standard deviations in each time
O
m
range.
Table entry in Student's "T" table for a given
t
confidence level.
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