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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UCL 169.7 11633 LCL =11 57.0 t150 을 1100 052 1000 2229 505 6471 Year (1·1936) 109 75 UCL 410 R =23.9 LOL-6.8 1522 29 36 43 57 E471 Year (1-1936)