The average for this week's deliveries of a page is first calculated. The deliveries with less than the average delivery time are again averaged. This average is the "fast" delivery time. Similarly, the "slow" delivery time is the average for deliveries slower than the weekly average. The difference between these numbers (the "spread") gives a measure of the variability in delivery.
This kind of double averaging is not usually recommended in statistics textbooks. One "rogue" slow delivery will shift the "slow" average noticeably. In this case, we think this well represents how your clients see you server. One "rogue" slow delivery will stand out more in their minds than several on time deliveries.
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