Deviation Reduction with Histograms

Deviation (i.e., variation) is one the three silent killers of productivity and profitability. While more commonly used in manufacturing, both service and manufacturing companies can use histograms to eliminate deviation. In manufacturing, if a part is too big or too small, too thick or too thin or whatever, it can cause waste, scrap and rework. In manufacturing, we might also want to know how long a part or product will last and when it will most likely fail. In a service industry, variability centers around cycle times to deliver the service: wait time at a fast food restaurant, emergency room or automobile service counter.

Histograms for Reducing Manufacturing Variation

Manufacturing companies focus on reducing variation in the finished product, because excessive variation leads to scrap and rework either at the plant or at the customer's location. Histograms help show the amount of variation. The first step is to measure a sample, in this case the bursting strength of bottles, often in subgroups of five:

histogram data in excel

Then, use the QI Macros to draw a histogram with USL = 346 and LSL = 200:

histogram with values outside of the spec limits

There are bottles outside of the USL and LSL. Bottles with too high of a bursting strength may use excessive material which increases costs. Bottles with too low of a bursting strength may cause problems for the customer. The measures of process capability, Cp and Cpk, are both less than one, so the process isn't capable. Teams can now do root cause analysis to reduce the variation.

After analysis and improvement, measure the variation again:

capable process with all data points on the histogram inside the spec limitsNow the bottles fit between the USL and LSL, but Cp and Cpk are slightly less than 1.33 (4 sigma) which is a standard for most manufacturing companies. More improvement may be necessary.

We could also show these two charts side-by-side to show the improvement using the QI Macros histogram template:

side by side histograms showing process improvement

You can easily create breakthrough improvements in variation using these tools and the QI Macros for Excel.

Master these tools and you'll be well on your way to creating breakthrough improvement with Six Sigma. QI Macros will support you every step of the way.

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