ACCURACY AND PRECISION

  

Measurement System Analysis

Chapter 6

Fundamental properties of a GOOD Measurement System

• ADEQUATE SENSITIVITY     

        10-1 Rule

Instrument Accuracy  ( and therefore Least count  ) should divide the tolerance ( or process variation ) into 10 parts or more. 

• Measurement System ought to be in statistical control - presence of only common causes of variation and not the special causes.

Accuracy and Precision 

* Accuracy tells how much the points are closed to the target.
Measurements are said to be accurate if their tendency is to center around the actual value of entity being measured.

* Precision tells how much these points are closed to themselves.
Measurement are precise if they differ from one another by small amount.

* In a Measurement System both Accuracy and Precision  are required to reduce measurement variation.

* Accuracy and Precision are not the same.

* Variation in various conditions

Calculating Accuracy and Precision : 

 1) Accuracy 

       * Calculate % Error   

       *

       * Lesser the % Error of Measurement,higher is the accuracy                        

 2) Precision

       * To determine if a value is precise find the average of your data then substract each measurement from it.This gives you a table of deviations.Then , average the deviations.This give you a value called Uncertainty. 

       * A plus or minus value says how precise a measurement is 

       * Deviation = Average - Actual 

       *         

          

       * " Lesser the Uncertainty , Higher is the precision "

   

Accuracy and Precision Error : 

• If the measurement system has accuracy error,then it will create location error in the result of measurement.

If measurement system has Precision error, then it will create spread error in results of measurement.

Note : Accuracy and Precision are not the same.