11. WORKLOAD CHARACTERIZATION › 11.6 Case Studies › 11.6.1 Batch Job Class › 11.6.1.3 Report Interpretation › 11.6.1.3.1 Descriptive Statistics Report

11.6.1.3.1 Descriptive Statistics Report

The Descriptive Statistics Report provides a summary of the
descriptive statistics calculated for the cluster features.
This report is used to evaluate the effects of trimming 2.5%
(or the specified value) off the right-hand tails of the
distributions on the sample averages and standard deviations.
A sample report is shown in Figure 11-25.

In Section 11.4.3 we discussed trimming a small percentage of
the right-hand tail from the distribution to minimize the
estimate of the standard deviation that was made for the
population.  For example, if the average value for CPU time
in the full sample is 16.6 with a standard deviation of 175.1
seconds, then the coefficient of variation (that is, the
standard deviation divided by the mean) is 10.55.  As a
general rule, a high coefficient of variation (that is,
greater than 2.5) indicates that the outliers are still
present in the trimmed distribution.

If, for example, 2.5% of the observations are excluded from
the calculation of the sample statistics, the average and the
standard deviation would be 5.8 and 11.1 seconds,
respectively.  These values result in a coefficient of
variation of 1.9.  In this batch job class study, the
statistical behavior of the observations would be
significantly improved by excluding 2.5% of the observations.

If the coefficient of variation for one of the features
exceeds the recommended range, you should decrease the
percent of the distribution to be included in the trimmed
statistics by adjusting Observation trimming (percent) on the
Statistical Analysis Parameters screen shown in Figure 11-22.

                                                        DESCRIPTIVE STATISTICS

                        NUMBER OF OBSERVATIONS:  2000
                                                        ----------SAMPLE----------  ------TRIMMED 97.50%------

                        FEATURE      MIN        MAX      AVERAGE    STD DEV    CV    AVERAGE    STD DEV    CV
                        --------  ---------  ---------  ---------  ---------  ----  ---------  ---------  ----
                        JOBMXNTA          0          6     0.2135    0.54687   2.6   0.154951   0.391913   2.5
                        JOBTCBTM       0.01    7115.71    16.6509    175.139  10.5    5.80658    11.0867   1.9
                        JOBNLR            0     334275    2327.42    12277.9   5.3    1019.77    2339.31   2.3

 Figure 11-25.  Descriptive Statistics Report