Monitoring weight. Joe has recorded his weight, measured at the gym after a workout, for several years. The mean is 181 pounds and the standard deviation is 1.7 pounds, with no signs of lack of control. An injury keeps Joe away from the gym for several months. The data below give his weight, measured once each week for the first 16 weeks after he returns from the injury:
Joe wants to plot these individual measurements on a control chart. When each "sample" is just one measurement, short-term variation is estimated by advanced techniques.13 The short-term variation in Joe's weight is estimated to be about σ = 1.6 pounds. Joe has a target of μ = 181 pounds for his weight. Make a control chart for his measurements, using control limits μ ± 2σ. It is common to use these narrower limits on an "individuals chart." Comment on individual points out of control and on runs. Is Joe's weight stable or does it change systematically over this period?