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I'm trying to create a series of control charts to monitor performance in customer service over time based on survey results.
We collect about 30 responses a week to each of the attribute questions (eg. "Customer service provided me with all the information I need.") on a 5 point satisfaction scale.
When I try to create an X-bar control chart, the mean plots correctly but the Upper and Lower control limits are not straight horizontal lines like the centerline. They seem to vary to reflect the variance of each weekly sample instead of for the whole data set.
Does anyone know how to get the UCL and LCL for the whole data set on the chart?
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