![]() ![]() Control charts are constructed by plotting samples of your process output collected over time. There are several types of control charts, and the one that you choose depends on the type of output that your process produces and the sampling plan that you use to collect it. Tracking variables. If you record process parameters or things that you can measure, such as temperature, diameter, or flow rates, then you should use one of the variable charts. They tell you how your process is running. Specifically, they compare the actual machine readings to the set point or target values, and they tell you how well your finished product meets specifications. The available variable charts, according to sampling plans, are as follows:
Tracking attributes. If you want to track defects, defective units, or other things that you can count, such as defective parts, cracks, bubbles, or late shipments, you should use an attribute chart. They classify defects into categories so you can monitor a specific defect type. They also tell you types of defects and how they occur. The available attribute charts, according to the type of attribute tracked and sampling plan, are as follows:
Multiple charts can be used for process control. For example, a radio manufacturer can use a u chart to track cosmetic defects and a p chart to track units that don't work. General guidelines for assessing process control. From these plots, you can determine whether the process is functioning within prescribed limits. Random variation within those limits indicates the process is stable. Nonrandom variation indicates that something is wrong. Any of the following symptoms in a control chart indicate a possible unstable process:
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Control Chart Principles |