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High-Performance IT Services by Terry Critchley

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Appendix IV: Some Performance Math

General Math

Sums and Products

Summation: The symbol Σ (Greek upper case sigma) denotes a summation of items. The example below should make the function clear.

a1+a2+a3+an=i=1nai

Product: The symbol Π (Greek upper case pi) denotes a product of items. The example below should make the function clear.

a1×a2×a3×an=i=1nai

Mean Values of Quantities

The use of means or averages of some sort are common in IT since very few measurable parameters always have a fixed value. There are mean response times, mean reliabilities, average utilizations, and so on, often depending on mathematical distributions for their derivation and calculation.

Arithmetic Mean (Average)

This is the most common type of average. If we have ...

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