Control Chart

Skill level: Advanced

Description

A control chart is a visual representation of a series of data that have been collected over a certain period of time. It offers a graphical view of the behavior of the process you are monitoring. The data are generally collected on the key variable inputs rather than the output.

Benefits

  • Qualitative representation of the process under observation, therefore based on data and not intuition
  • Pro-active approach to tracking process performance using the key inputs allows early corrective or preventive actions
  • Easy to detect sudden variation or abnormal behavior over time
  • Shows how cycle, period, season, or time affects overall performance

How to Use

  • Step 1.  Collect the data to be analyzed.
  • Step 2.  Enter data into a spreadsheet tool such as Microsoft Excel, formatting to accommodate the creation of a chart or graph.
  • Step 3.  Create the chart using a statistically oriented application. To assist with analysis, there are different ways of looking at the data, including averages, medians, standard deviations, etc.
  • Step 4.  Analyze the control chart to determine if the process is in control and stable.
  • Step 5.  If trends indicate an existing or potential problem, implement changes to the process and continue to monitor.

Relevant Definitions

Control limits: Values calculated from the data, showing the possible range of variation around the mean (3 sigma).

Mean: Average value of the data set.

I-MR: Individual and moving range (variable data).

Xbar-R: Average and range (variable data – for between and within).

Example

A manager in the accounting department wants to track how long it takes to process and approve expense reports on a weekly basis. Because the expense reports are batch-processed as opposed to individually processed, each person in the department tracks the number of hours per day and the number of reports processed during that time.

Key inputs:

  • Number of reports processed
  • Total time in minutes spent to review and approve the reports

These inputs will provide insight into the average time spent per employee on each report. To display this information visually, an Xbar-R chart type will be used.

First, the data must be captured. The data set is shown in the table below.

Control_Chart_Table1

These data generate the control chart below.

Control_Chart_Figure1

The chart indicates that the process is operating within acceptable limits. The process should continue to be monitored for an additional period of time to determine if there are variations by day, month, etc., that would indicate continual improvement opportunities.

 

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