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In the previous article we have outlined the three main features of a statistical distribution: Location, Spread and Shape.

We also defined the term "Measurement" and its possible definition domains "Discreet" and "Continuous".

When we talk about Statistical Process Control (SPC), in the case of Continuous measurements (variable data), it is common to speak of Average as a statistical location, Range and Standard Deviation as spread statistics.

Control Cards are the tools used to track these statistics.

There are several types of Control Cards, we will see the main ones in the case of Continuous Measurements: **X-Chart** for Single Measurements (n = 1), **R-Chart** (2 ≤n ≤ 9), **S-Chart** (9 <n≤25)

Choosing a Control Card rather than another depends on the number of measurements that make up the sets (subgroups).

Each control card always analyzes the two statistics, Location (Average) and Spread (range or standard deviation).

The measurement system to lead to statistically reliable data must be:

Accurate: The numeric value attributed to the property must be "close" to reality;Repeatable: Can one person, using the same gage, measure the same part consistently?Linear: Accurate and repeatable measurements must be linearly distributed across the range of possible results;Reproducible: Can two people using the same gage, measure the same part consistently?Stable: measurements on the same object produce the same results if done in the future as in the past.

Used in case the particular process does not allow to perform consecutive measurements (quality control with destructive testing, too high cycle time, etc.)

In the case where the number of subgroup measurements are limited (2 to 9), it is common to use the "Average" and "Range" statistics pair for the control card.

Used in case the number of measurements is greater (9 <n <25). You use the Standard Deviation (Sigma) instead of the Range because with an high number of measurements the standard deviation is more efficient.

Click here for downloading an Excel file with examples and templates.

In the next article we will analyze the case of discrete measurements (count of attribute).

Process Excellence Lead & Founder of Lean Six Sigma Community

Management engineer with 15+ years of experiences in multinational companies, employed in different roles but with common goals: Continuos Improvement, processes standardization & optimization.

I'm trainer and consultant about Lean & Six Sigma methodologies, I love to help organizations to improve their performances in terms of times/costs/defects/spaces reductions.

I founded Lean Six Sigma University & Community, two synergic projects developed for creating a common place where learn and divulgate knowledges about Lean and Six Sigma methodologies applied to different ambits.

The application of Lean & Six Sigma methods are not simply methods learned and applied, it is a cultural change.

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