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given in tabulations such as this is the basic material used in investiga- 

 tion work relating to control of quality. The composite summary at 

 the bottom of the figure is used directly for computing rates. 

 The quality report for this product is based on the following: 



(1) The data are obtained by the check inspection of representative 



samples of the total product. 



(2) The average quality for the base period is taken as the " standard 



expected " quality for current product. Control lines are thus 

 placed above and below the base period rate of 0. 



(3) The limits are computed each month on the basis of the number 



inspected during that month, using a probability value of .997. 



The computations shown below the rating chart of Fig. 6 indicate 

 the work necessary to the determination of the control limits for a 

 given month. The basis of these computations is given in the Ap- 

 pendix. The limit lines on the chart are broken lines merely because 

 the number inspected varies from month to month. 



Fig. 7 — Rating chart with constant control limits 



When production and inspection schedules are fairly uniform so 

 that the number inspected per month remains substantially constant, 

 the limits may be computed once a year using some estimated size of 

 monthly sample and drawn as parallel lines across the chart as in 

 Fig. 7. This approximation is justified when the loss of accuracy there- 

 by introduced is outweighed by the extra charting costs associated 

 with monthly computations of limits. 



Example II — Monthly Rates Showing Quality of a Product Before and 

 After a Screening hispection 

 Assume the following procedure to be in force. 



(1) The shop product is inspected 100 per cent by an inspection group 

 which serves as a screening medium for eliminating defective 

 units. 



