Shown below are an x chart (top) and an R chart (bottom) obtained by using the monthly elevations of Lake Mead at Hoover Dam (based on data from the U.S. Department of the Interior). The elevations are in feet above sea level. The control charts are based on the 12 monthly elevations for each of the 75 consecutive and recent years available as of this writing. What do the control charts tell us about Lake Mead?
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