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CR: Histogram bars beyond specification lines

Revision Date: 2005-09-19

Problem:

Occasionally, SQCpack and CHARTrunner will show histogram bars beyond the specification lines. This may happen despite the fact that a careful review of the data shows no values beyond the specifications. How can this be?

Solution:

Consider that each bar on a histogram is like a bucket. As the software evaluates each data point, it has to decide which bucket to throw the data into. Imagine that each bucket has a label on it that reads something like this: in this bucket put each value that falls between 5.1 and 5.5. The size of each bar on the histogram is a count of the number of data values that were put into that particular bucket.

Next, imagine that your upper specification is 5.4. The last bar (on the right) might show that 3 data points are in this bucket. Well, these three data points might be 5.1, 5.2, and 5.3 – ALL of these are below the upper specification, however the last bucket on the histogram is for data points up to 5.5. This is what can make a histogram bar appear beyond the specifications.

If this is unflattering to your process, you may want to use the custom cells feature. This allows you decide how many buckets to use and what label to put on each bucket. By adjusting these values, your histogram bars can usually be made to appear within the specifications – assuming your data is within the specifications.

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