Tuesday, October 9, 2007

3.Histogram and the Normal Distribution

A good way to represent a situation is to transform data into a picture; the histogram. It represents the distribution of this data.
In most of the cases, these distributions are similar to the normal distribution represented by the bell shape curve. The normal distribution makes it possible to compute, with a high degree of confidence, the probability of defects._
Shape, Location and Spread

The majority of process data will be normally distributed (Bell shape curve). If the histogram is not bell shaped, the data should be investigated further .

The term "Measures of Location" refers to the various ways in which the central tendencies of the data distribution can be calculated and displayed.

Generally, the value of the mean (average): x is the best way to estimate the central tendency of the most common distributed data.
The third important characteristic of a distribution is the spread (variation).

Generally, we estimate this variation by calculating: The range (R), or The standard deviation (s or s)._

Specification Limits

In the manufacturing environments, specifications are usually set by the Engineering functions. Operators have a clear set of limits to work to, usually set out on engineering drawings, and when the output of their process is above the Upper Specification Limit (USL), or below the Lower Specification Limit (LSL), then we say that a defect has been produced._

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