Monday, September 16, 2013

Understanding I-MR Charts for Process Improvements

Ever since my first exposure to statistics, I-MR charts have been my absolute favorite. It is a simple yet powerful tool for understanding process performance and identify opportunities for process improvement.





This is an I-MR chart for training feedback. The Individual value plot at the top plots every value separately against a time range whereas the Moving Range plots the difference between two points

I-MR chart gives us the following facts about our process:
  • Plot the Individual data points against time
  • Derive the control limits of the process (Max and Min performance)
  • Mean performance of the process (Typical performance of my process)
  • Variations in process (in Moving Range chart which is difference between the consecutive data points) explains how much the process performance fluctuates in the given scenario. More the fluctuation, less predictable my process is. 
  • When has my process behaved abnormally and hence I should do a root cause analysis on those instances (Outliers)
I-MR charts are a powerful tool for process improvement and can be used as follows:
  • Understand process behavior and baseline the same
  • Identify opportunities to improve process performance by
    • Identifying special causes behind the outliers
    • Comparing the control limits against the specification limits or expected performance values
    • Identifying causes behind the variation in process by identifying data points when substantial variation has occurred
  • Post improvement, re-baseline the process behavior and compare the performance improvement by comparing the old and new mean as well as variation