Question

discuss the purpose of control charts, how they are created, what calculations are used, and why they are important in measuring quality. Then, create a control chart based on criteria that you come up with and explain it.

Answer 1

Control charts are graphs that are used to study the changing of processes over time. They can also be used to predict the future performances of the said process. Control charts are mainly used for time-series data and logically comparable data. The data are plotted in chronological order.

There are two kinds of control charts:-

• c control chart: where ‘c’ represents no. of non-conformances within one subgroup
• u control chart: where ‘u’ represents no. of non-conformances per unit within one subgroup

A control chart consists of an upper line for upper control limit (UCLc), a lower line for lower control limit (LCLc) and a central line for the average. The UCLc and LCLc can be calculated as follows:-

where,

Example:-

Suppose we have to track the no. of burgers sold by a fast food joint on a monthly basis. The fast food joint is the area of opportunities for sales to occur. We have to collect the data each month on the no. of burgers sold. Once, enough data is available, we can calculate UCLc and LCLc.

Month	No. of Burgers (c)
Jan-17	220
Feb-17	157
Mar-17	170
Apr-17	135
May-17	142
Jun-17	178
Jul-17	241
Aug-17	163
Sep-17	121
Oct-17	194
Nov-17	172
Dec-17	219
Jan-18	156
Feb-18	145
14	2413 ----> Total

c bar = Sum of no. of burgers/ no. of months = 2413/14 = 172.36 UCLc = 172.36 + (3* (square root of 172.36)) = 211.74

LCLc = 172.36 - (3* (square root of 172.36)) = 132.97

There should be two lines drawn on graph - one at UCL = 211.74 and the other at LCL = 132.97.

Distinct causes of variation are detected on control charts by observing definite types of patterns that appear on the control chart. The points beyond the UCLc and LCLc is one such pattern. These kinds of patterns indicates that something has happened to cause the process average go up or down, that is, a special cause is present. Identifying patterns and what they refer to in the process is one crucial to reason for special causes. All the control chart patterns that form on the control chart indicate special causes of variation that are present.