The AFTER performance also passed all SPC tests so the new control limits should be used going forward to monitor this process. With the statistical evidence that a shift has occurred in the mean from 19.65 minutes to 9.79 minutes. The test statistic of 26.42 is greater than critical t-value at 0.05, and dF = 76 which is 1.67 for one tailed test. There are a couple ways to conclude this. To statistically analyze whether the mean has changed you could use the 2 sample-t test or paired-t test (depending on the data and assuming the data is normally distributed). You can see from the chart the average for the individual measurement times went down to 9.79 minutes and, by examining the lower chart, you can see the variation among the times also was reduced. Time is an continuous data type that would you an SPC chart such as an I-MR.
The times were charted with each time representing its own group (subgroup size = 1).
The objective of the team is to eliminate or explain all special cause variation and make fundamental, unprecedented improvements to drive the existing level of common cause performance to a reduced variation and more precise performance around a target.īelow is an example of data compiled at the end of the IMPROVE phase from a time study before and after the improvements were implemented on an inspection process. If this were the previous (BEFORE) data of a process, and all the variation is explained by common cause inherent variation then it will take a fundamental change (hopefully an improvement) to change and sustain this performance. If this were the new (AFTER) data from a process improvement and this performance is better and more desirable than the BEFORE performance, then these control limits could be set as the new process control limits. This would suffice for the stability portion of an MSA. Keep in mind there are several estimates for sigma (standard deviation) and each use should be agreed upon with the customer and the reasoning for its selection.īoth charts indicate a process that is stable and in control. The calculation applies the short-term estimate with un-biasing constant since it is most likely a sampling representing the short term performance of the process. The data below of measurements were taken from the overall length of 30 different widgets. There are many types of tests that can determine control and points within the control limits can also be out of control or special cause. I-MR charts should be in control according the control tests that you elect to use.
However, this value is adjustable in most statistical software programs. Therefore there will be one less data point in the MR chart than the Individuals chart. Each data point stands on its own and the means there is no rational subgrouping and the subgroup size = 1.Ī couple other common charts used with subgroups >1 are:Ī typical Moving Range (MR) Chart uses a default value of 2, which means each data point plots the difference (range) between two consecutive data points as they come from the process in sequential order. The Individuals (I) Chart plots each measurement (sometimes called an observation) as a separate data point.
This chart is used to plot CONTINUOUS data. I-MR charts plot individual observations on one chart accompanied with another chart of the range of the individual observations - normally from each consecutive data point. Share Facebook Twitter I-MR Charts Individuals - Moving Range Charts