QC1 LEC PRELIM: Statistical Quality Control

Statistical Accuracy

The agreement of the mean value of a series of experimental results with the true value

Absolute Error

 mean - true value

Relative Error

absolute error/ true value

P Chart

Control chart for fraction defective

Statistical Precision

• measure of reproducibility of data within a series of results

• not necessarily accurate 

Average Deviation

Calculated by finding the differences between individual results and the mean

Relative Average Deviation

is found by dividing the average deviation by the mean and multiplied by 100 as % or 1000 ppt

Standard Deviation

sd or s

Standard Deviation

preferred measure of precision 

Standard Deviation

It shows how much variation or "dispersion" there is from the average (mean or expected value) 

Standard Deviation

low sd indicated that the data points tend to be very close to the mean and vice versa

each score

What does this symbol mean: x

no. of values

What does this symbol mean: n

Summation

What does this symbol mean: ∑

Relative Standard Deviation

• RSD 

• Coefficient of Variation 

Relative Standard Deviation

Dividing the standard deviation by mean and multiplied by 100 

Range

Difference between largest and smallest value 

Statistical Quality Control

Makes use of control charts, a tool which may influence decision related to the functions of specifications, production or inspection.

Variable Chart

Use actual records of numerical measurement on a full continuous scale such as meter, grams, liter

• Ex: Mean and R charts

Attribute Chart

Makes use of discrete data classifying the number of items conforming and fallin

• Ex: P Chart

UCL and LCL

2 horizontal parallel lines on either sides of the solid line which indicate limits 

Upper Line

Normally 3 standard deviations above the center line

UCL

 Determine the Formula: Mean + 3 (SD)

LCL

Determine the formula: Mean - 3 (SD)

Lower Line

3 standard deviations below the center line