18.21.
18.21.1.
Principles. The industrial quality data are of two types :
Attributes : Number of welds rejected, number of defective holes, number of defects on an ingot or sheet, number of defects of various types on casting, etc.
Variables : Length, weight, diameter, percent carbon, hardness, thickness, ash content, pressure, acidity percent, etc.
Variation is inevitable. No two two pieces are alike. Even pieces produced by most modern machine can be shown to differ if inspected by a corresponding precise instrument.
The variation consists of two parts—natural or inherent due to chance causes in the process and unnatural or external due to assignable causes that can be traced. Distinction between chance and assignable causes of variation is made in Art. 18.22. Target analogy is given in Fig. 18.11.
Variation due to chance plus assignable causes
Variation due to chance cause only
Fig. 18.11
Variation resulting from natural causes alone reveals certain pattern or follows certain statistical law—such as posson, normal, etc. Reference may be made to the pattern of variation in thickness of steel sheets, number of machines under breakdown, etc.
From known pattern or population distribution, the pattern of distribution of samples drawn from it can be predicted.
