However, the scattered read wait time of 0.093 ms occurs much more frequently than the other waits. A more accurate average calculation would take into account the occurrences of the scattered read waits in addition to its average event wait time. This is called a weighted average, and it is a much better average calculation when working with diverse and highly variant data sets, as we have here.

The average calculation, weighted by the total wait time (which includes the weight occurrences) is shown in Figure 9-21. It is calculated as follows:

If you think about it, the weighted concept makes sense. Because the scattered read waits happen so much more often, the average IO read time should reflect this and be pulled toward the scattered read wait times. As Figure 9-21 shows, the weighted average value actually rounds to the average scattered read wait time of 0.093. While the difference may seem insignificant, not only can this have a dramatic effect when anticipating the impact of a performance solution, but it also makes the averages more realistic.