Firstly, considering the X-R charts in Figure 1, we see that the distribution is stable over the period of study (this may also be reported via Minitab s Capability Sixpack ). Suppose the LSL = 37, the USL = 43, and our target for this process is midway between the specs, i.e. Example Consider the following simulation. If the Normal distribution assumption is not appropriate, yet capability indices are recorded, one may seriously misrepresent the true capability of a process. We therefore need to determine whether the underlying distribution can indeed be modeled well by a Normal distribution. This is particularly important to remember when performing capability analyses. The Normal Distribution One should note that there are an infinite number of distributions which may show the familiar bell-shaped curve, but are not Normally distributed. Using Exponentially Weighted Moving Average (EWMA) Charts, Asia-Pacific Engineer.ΔΆ 2. Introduction to Statistical Quality Control, 3rd Edition. For more information on these charts, the interested reader is referred to Montgomery 1 and an example by Bower 2.
Two types of charts are primarily used to detect smaller shifts, namely Cumulative Sum (CUSUM) charts and Exponentially Weighted Moving Average (EWMA) charts. of the order of 1.5 standard deviations or above. Xbar R, Xbar-S) are sensitive to detecting relatively large shifts in the process, i.e. The most frequently used form of control charts in operation today are those which have their derivation from the pioneering work of Dr. In order to assess whether or not a process is in statistical control, quality practitioners use control charts. Control Status If the process is not in statistical control we are unable to reliably use our estimates for spread and location, hence our formulae are redundant. One finds in practice that, typically, one or both of these assumptions are disregarded.
If these assumptions are not met, the resulting statistics may be highly unreliable. The distribution of the process considered is Normal. Assumptions There are two critical assumptions to consider when performing process capability analyses with continuous data, namely: 1. Procedures when dealing with non-normal data will be considered in the following edition of EXTRAOrdinary Sense. It is the aim of the author to address such issues by the use of discussion and case studies, and to provide some useful guidelines and insights when performing capability analysis using Minitab. It is important to emphasize that there are certain crucial assumptions, which allow the use of such values to have a meaningful interpretation, which are frequently overlooked. Abstract The use of capability indices such as C p, C pk, and Sigma values is widespread in industry. 1 Process Capability Analysis Using MINITAB (I) By Keith M.