Using CitectSCADA > Understanding Statistical Process Control > Process Variation

Process Variation

To use SPC effectively, understand the concept of variation. When a product characteristic is measured repeatedly, each measurement is likely to differ from the last. This is because the process contains sources of variability.

When the data is grouped into a frequency histogram, it will tend to form a pattern. The pattern is referred to as a probability distribution and is characterized in three ways:

Note: Most SPC techniques assume that the collected data has a normal distribution.

Variation is generally categorized into one of two types:

Statistics indicate that special variations account for about 15% of departures from process quality requirements. Typically these departures require local action (equipment repair and so on) for solution.

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