![]() ![]() This is shown in the figure below.ĭ 4 and D 3 are control chart constants that depend on subgroup size. If a subgroup range is beyond the control limits, it is a signal that something has changed for that subgroup range. Each subgroup range is compared to the average range to determine if a significant change has occurred. The range chart monitors the variation between results within a subgroup (often called the within-subgroup variation). The X chart monitors the variation in the average bag weight from hour to hour. What is the X chart monitoring in Plan A for the bag weight example? X is the overall average of the X values, R is the average range, UCL is the upper control limit, LCL is the lower control limit, and A 2 is a control chart constant that depends on subgroup size. The variation being monitored is shown in the figure below. If a subgroup average is beyond the control limits, it is a signal that something has changed for that subgroup average. Each subgroup average is compared to the overall average to determine if a significant change has occurred. The X chart monitors the variation in subgroup averages over time (often called the between-subgroup variation). What is the variation being monitored on the X and R chart? The range of each subgroup is found and plotted on the range chart. The average of each subgroup is found and plotted on the X chart. Remember, some thought has to go into how you form the subgroups. With the X-R chart, samples are taken, and a subgroup is formed. The two sampling plans are shown in Figure 1. First, the variation examined on each chart is discussed. But is one better than the other? We will see. In this case, either the X-R chart or the X-mR chart will work. Do you know why the X-mR chart is not good for this sampling plan?Īnother option is to take a sample every 15 minutes (Plan B). In this case, the X-R chart is the best option. There are numerous ways you could do this including taking four consecutive samples at the top of each hour (Plan A). You want to take four samples per hour to monitor the variation. Too much and you are giving away product too little and you may get customer complaints that the bag is not full enough. You are in charge of a line that loads a product into bags. Understanding the Variation Being MonitoredĬontrol charts, when properly designed, monitor variation over time. You can download a pdf copy of the publication at this link. Please feel free to leave a comment at the end of this publication. Understanding the Variation Being Monitored.A simulation is used to show why this is true. This publication compares the X-R chart with the X-mR chart to better understand the differences – and to show that the X-mR chart actually can give more false signals than the X-R control chart. Yes, it usually works – but is it the best choice always? Some say the X-mR chart is the only chart you need to know because it always works. If you have infrequent data (like monthly), you don't have many options beyond the X-mR chart. It has probably given way to the X-mR chart as the use of SPC spread beyond manufacturing into other areas and "frequent" data became less frequent. The X-R chart, at one time, was the most used control chart. To explore this concept, two control charts will be examined and compared in this publication: the individuals (X-mR) control chart and the X-R control chart. You just need to be sure it is a movie of the variation you are interested in examining. What variation do you want the control chart to analyze/monitor? Every control chart is a movie of the variation in your process over time. There really should be some thought given to how a control chart is designed – and that depends on what you want to obtain from the control chart. What do you do? Many of us simply put the data into a software package and select the control chart we want to use – and, presto, we have our control chart. You want to analyze it using a control chart. Select this link for information on the SPC for Excel software.) Select "Return to Categories" to go to the page with all publications sorted by category. (Note: all the previous publications in the variable control charts category are listed on the right-hand side. ![]()
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