Long range heterogeneity error

Long Range Heterogeneity (or Point Selection) Error errors of this type are nonrandom and arise from existence of spatially localized hot spots in the sampling target such errors can be identified and reduced by taking many sampling increments to form the final sample. 

Error 3 Long-range nonperiodic heterogeneity fluctuation error (process variation)... 

LONG-RANGE NONPERIODIC HETEROGENEITY FLUCTUATION ERROR... 

The previous errors addressed heterogeneity on a small scale. Now we examine heterogeneity on a large scale the scale of the lot over time or space. The long-range nonperiodic heterogeneity fluctuation error is nonrandom and results in trends or shifts in the measured characteristic of interest as we track it over time or over the extent of the lot in space. For example, measured characteristics of a chemical product may decrea.se due to catalyst deterioration. Particle size distribution may be altered due to machine wear. Samples from different parts of the lot may show trends due to lack of mixing. 

The long-range periodic heterogeneity fluctuation error is the result of nonrandom periodic changes in the critical component as we track it over lime or over the entirety of the lot in space. For instance certain measurements may show periodic fluctuations due to different control of the manufacturing process by various shifts of operators. Batch processes that alternate raw materials from two different suppliers may show periodicity in measurements. 

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