Tham khảo tài liệu 'engineering statistics handbook episode 3 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | . Monitoring short-term precision ENGINEERING STATISTICS HANDBOOK ÍŨCME TOOLS ẬAIOỈ SEARCH BACK nextI 2. Measurement Process Characterization . Statistical control of a measurement process . How is short-term variability controlled short-term precision Monitoring future precision Once the base line and control limit for the control chart have been determined from historical data the measurement process enters the monitoring stage. In the control chart shown below the control limit is based on the data taken prior to 1985. Each new standard deviation is monitored on the control chart Each new short-term standard deviation based on J measurements is plotted on the control chart points that exceed the control limits probably indicate lack of statistical control. Drift over time indicates degradation of the instrument. Points out of control require remedial action and possible causes of out of control signals need to be understood when developing strategies for dealing with outliers. Control chart for precision for a mass balance from historical standard deviations for the balance with 3 degrees of freedom each. The control chart identifies two outliers and slight degradation over time in the precision of the balance Control chart for precision of balance Standard deviations with 3 degrees of freedom plotted vs year TIME IN YEARS Monitoring where the number of measurements are different from J http div898 handbook mpc section2 1 of 2 5 1 2006 10 11 29 AM . Monitoring short-term precision There is no requirement that future standard deviations be based on J the number of measurements in the historical database. However a change in the number of measurements leads to a change in the test for control and it may not be convenient to draw a control chart where the control limits are changing with each new measurement sequence. For a new standard deviation based on J measurements the precision of the .
