Question

Our text reading this week describes two forms of control chart designs: a) those based on X-bar and R, and b) those based on T and sigma-naught. The formulas for building these charts are virtually the same, and so we can expect the charts to look fairly similar. Discuss the appearance and usage you expect to see for a given process charted using both techniques. What are the similarities and differences? Are the differences affected by the underlying quality of the process being charted? Are there advantages of one approach over the other if our goal is to improve?

Answer 1

There are some basic differences between those who back control charts (X-Bar) & those who back pre-control charts (T-Based). The pre-control supporters view any commodity within the specification as being of equivalent good. All results are regarded as ‘good’ / ‘bad’ & the dividing border is a distinct cliff. The part which hardly meets requirement is as good as the part that is absolutely concentrated on the target value (T). Manufacturing commodity tighter than the specification boundaries is considered as being an unnecessary cost. This technique concentrates on the voice of the consumer in that the pre-control boundaries are based on the upper & the lower specification boundaries (USL & LSL). These limits are selected so that the hard stop border to pre-control graphs are at the consumer specification & cautionary borders are at ±50 % of the specification.

X-Bar control limits are selected so that time is not lost looking for unwarranted concern . The aim is to take act only when required. Control limits are computed by obtaining the standard deviation for the sample data altered for size of sample & multiplying that numerical by 3. That numerical is then added to the mean for the UCL & subtracted from the mean for the LCL. Shewhart provided us certain constants that simplify these computations. The control chart tests are structured to flag those points which aren’t performing ‘normally’

The chart concentrates on the variation that is on account of the procedure itself. Control borders are created from the process data & not linked to specification boundaries. This is called a voice of the process since the process is giving info about itself.

Pre-control measures comply with the consumer specification, the voice of consumer. Control charts measure process variations. Control charts give power in evaluation of a process specially while utilizing rational subgrouping. The Rational subgrouping lessens the likelihood of false positives; which isn’t possible with the pre-control charts.

Pre-control charts have restricted utilization as an improvement instruments . These don’t sense the shifts, the drifts & the tendencies with the statistical assurance as the control charts do.