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Figure 7-9. Simple control chart of annual earthquakes of magnitude 6.0 or greater
This version doesn't show any points “out of control,” or beyond three standard deviations
from the mean. If we change from YEAR to MONTH , then the control chart changes to show a
number of points above the 3 sigma line, including a sharp outlier in March 2011 corres-
ponding to the Great East Japan earthquake, as shown in Figure 7-10 .
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