4.0 Capability Analysis: How is it made?

These steps assume that variables data has been collected over time, and that a control chart and histogram have been completed. The control chart should show no special causes, and the histogram should reveal that the data is normally distributed.

In the example, the team examined the arrival time of couriers in comparison to the scheduled arrival time over a month. Since time is being measured, the data is variables. The team completed a histogram and found the data to be normal. An X- R control chart was also completed, showing no special causes of variation. Since a control chart must be completed before performing capability analysis, the calculations from the control chart can be utilized. The information taken from the control chart follows:

• The sample size used in the control chart, n

• The overall average, (from an chart) or (from an X-MR chart.)

• The average range,(from an chart) or (from an X-MR chart.)

The numerical specifications or requirements should also be known. Information for the example is shown below:

n = 5

= 10.00

= 4.653

USL = upper specification limit = 14 minutes

LSL = lower specification limit = 0 minutes (on time)

Steps:
>4.1 Sketch the distribution
>4.2 Calculate the estimated standard deviation
>4.3 Determine the location of the tails for the distribution
>4.4 Draw the specification limits on the distribution
>4.5 Calculate how much data is outside the specifications
>4.6 Calculate and interpret the capability indices
>4.7 Analyze the Results

See also:
>1.0 Capability analysis: What is it?
>2.0 Capability analysis: What does it look like?
>3.0 Capability analysis: When is it used?

The above article is an excerpt from the "Capability Analysis" section of Practical Tools for Continuous Improvement.