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Run charts


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Run charts

  1. 1. Run ChartsAdding the element of time will help clarify your understanding of the causes of variation in yourprocesses. A run chart is a line graph of data points organized in time sequence and centered on themedian data value. The patterns in the run chart can help you find where to look for assignable causes ofvariation.What can it do for you?Histograms or frequency plots can show you the general distribution or variation among a collection ofdata points representing a process, but one histogram or one frequency plot can not show you trends orhelp pinpoint unusual events. Sometimes, a normal-looking distribution will hide trends or other unusualdata. To spot those trends, the data must be considered in time order. Plotting data on a run chart canhelp you identify trends and relate them to the time they occurred. This will help you in your search forthe special causes thatmay be adding variation Unusual variation can hide in a frequency plotto your process.Run charts are especiallyvaluable in the measureand analyze phases ofLean Six Sigmamethodology.
  2. 2. How do you do it?1. Select a characteristic from one of your processes. This characteristic could be presenting a problem because excessive variation often drives it outside of specification limits, or it could be a cause of customer complaints.2. Measure the characteristic over time intervals and record the data. Note the time or the time period that is associated with each data point.3. Find the median data value. To do this, list the data values in numeric order. Include each data point, even if it is a repeat value. If the number of data points is odd, the median is the middle value. If the number of data points is even, the median is halfway between the two values nearest the middle. For example, if the collected data points were: 5, 1, 18, 8, 12, 9, the ordered values would be: 1, 5, 8, 9, 12, and 18. The middlemost values are 8 and 9. The median is the average of those values, or 8.5. (Remember, the numerically-ordered data is only for determining the median. The data must be plotted in time order on the run chart to be of any value.)4. Set up the scales for your run chart. The vertical scale will be the data values, and the horizontal scale will be the time. Make the horizontal scale about two to three times the distance of the vertical scale.5. Label the vertical scale so that the values will be centered approximately on the median and so the scale is about 1 ½ to 2 times the range of the collected data.6. Draw a horizontal line representing the median value.7. Plot the data points in sequence. Connect each point to the next point in the sequence with a line. int ta ns ns int ta ns ns of r of r of r of r er t fo er t fo er t fo er t fo po f da po f da ru ru ru ruSome special cause variation reveals s s mb mi mb mi mb mi mb mi o oitself in unusual run-chart patterns. nu r li r li nu r li nu r li er erThese clues can direct you in your mb mb we we pe pe Up Upsearch for causes. Count the number Lo Lo Nu Nu nuof runs. Runs are sequences of points 10 3 8 35 13 23 11 3 9 36 13 24that stay on one side of, either above 12 3 10 37 13 25or below, the median line. One way 13 4 10 38 14 25 14 4 11 39 14 26of counting the runs is to circle these 15 4 12 40 15 26sequences and tally them. Another 16 5 12 41 16 26 17 5 13 42 16 27way of doing this is to count the 18 6 13 43 17 27number of times the run-line crosses 19 6 14 44 17 28 20 6 15 45 17 29the median, and then add one. 21 7 15 46 17 30 22 7 16 47 18 30 23 8 16 48 18 31Compare the number of runs you 24 8 17 49 19 31count to the accompanying chart. 25 9 17 50 19 32 26 9 18 60 24 37 27 9 19 70 28 43Numbers of runs outside the range 28 10 19 80 33 48shown for the number of data points 29 10 20 90 37 54 30 11 20 100 42 59are statistically unusual. 31 11 21 110 46 65 32 11 22 120 51 70 33 11 22 34 12 23
  3. 3. Too few runs (below the lower limit) generally indicate that something cyclic is systematically shiftingthe process average. Example of too few runsToo many runs could point to a problem of consecutive, over-compensating process adjustments orindicate that the data points actually came from two sources with different process averages. Example of too many runs
  4. 4. Look for sequences of ascending or descending values. Seven or more continuously increasing orcontinuously decreasing points indicates a trend that is shifting the process average. When you arecounting points, ignore any points that repeat the previous value. Repeated values neither add to thelength of the run nor break it. Examples of seven or more ascending and descending pointsSearch for seven or more consecutive points on the same side of the median line or 10 of 11 points or 12of 14 or 16 of 20. (Ignore any points that are exactly on the median.) Such a sequence indicates thatsomething has occurred to shift the process average in that direction. Examples of seven or more ascending and descending points
  5. 5. A sequence of 14 or more data points alternating up and down suggests a variation related to sampling(such as one reading early in the day and one reading toward the end) or that the data is coming fromtwo sources with different process averages (such as from two machines making the same part.) Inlooking for up-and-down alternation, ignore any points that are exactly the same as the preceding point. Example of 14 or more alternating pointsA sequence of seven or more points with exactly the same value usually should signal you to look for aspecial cause. While it is possible that your process has improved to the extent that the existingmeasurement technique is no longer sensitive enough to measure variation, it is usually more probablethat a gauge is stuck or broken or that someone is making up the data. Example of seven points in a row with the same value
  6. 6. Now what?Run charts can be very valuable inhelping your search for sources ofvariation. They are easy to plot and easyto interpret. The sampling isuncomplicated, and there are no statisticalcomputations to make. They can also beapplied to almost any process or any data.On the other hand, they are not an instantindicator. They are best used for spottingtrends; short shifts in the process cannotalways be detected with run charts. Inaddition, special causes that producegeneral piece-to-piece variation will notbe readily detected on run charts.Finally, a simple run chart cannotestablish the natural capabilities of aprocess, so it isn’t possible to use one topredict what specifications a process canactually meet. To do that, you need tocreate a control chart, a run chart withstatistical control limits. Steven Bonacorsi is the President of the International Standard for Lean Six Sigma (ISLSS) and Certified Lean Six Sigma Master Black Belt instructor and coach. Steven Bonacorsi has trained hundreds of Master Black Belts, Black Belts, Green Belts, and Project Sponsors and Executive Leaders in Lean Six Sigma DMAIC and Design for Lean Six Sigma process improvement methodologies. Author for the Process Excellence Network (PEX Network / IQPC). FREE Lean Six Sigma and BPM content International Standard for Lean Six Sigma Steven Bonacorsi, President and Lean Six Sigma Master Black Belt 47 Seasons Lane, Londonderry, NH 03053, USA Phone: + (1) 603-401-7047 Steven Bonacorsi e-mail Steven Bonacorsi LinkedIn Steven Bonacorsi Twitter Lean Six Sigma Group