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Figure 7-10. The simple control chart showing monthly counts of worldwide earthquakes
Also note that the lower limit is not real. It's below 0, and it's not possible to have a negative
number of earthquakes recorded.
Let's see how a more rigorous approach can enlighten us further about whether any statistical
outliers are included in the data.
The rigorous method
We'll add a few extra elements to this version, including a “Moving Range” timeline that
shows the absolute value of the change from quake to quake, control limits calculated from
this moving range, and data points colored by their type—in range, outliers, trends, and
shifts.
Start with a new Sheet, and repeat Step 1 of the quick method described in the previous sec-
tion to create a basic timeline, duplicating SUM(Number of Records) and creating a dual-
axis plot with axes synchronized. The first set of Marks should be a line, and the second set
of Marks should be circles, as shown in Figure 7-11 .
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