Central Limit Theorem

Section 2 Variation Fundamentals

Lesson S02-07

Text: Section 2 pages 41 – 58

Duration: 30 minutes

The Central Limit Theorem suggests:

Averages from a stable process tend to follow a normal distribution, even if individuals are not normal.
The tendency for the averages to form a normal distribution improves as the sample size increases.

Next, we build on our conceptual understanding of Process Control and consider how we may make best use of the data collected.

There is usually concern pertaining to the amount of data that is required for SPC purposes. People responsible for understanding the process may propose to measure every unit produced (for the important characteristics).

The objective of SPC is to determine process stability or instability. SPC is not an indicator of process capability.

Process capability refers to whether parts are in or out of the specification limits. We will learn procedures for estimating process capability, but realize that SPC should not be used for capability assessment.