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The process capability index, or process capability ratio, is a statistical measure of process capability: the ability of an engineering process to produce an output within specification limits. [1]
The value 3.267 is taken from the sample size-specific D 4 anti-biasing constant for n=2, as given in most textbooks on statistical process control (see, for example, Montgomery [2]: 725 ). Calculation of individuals control limits
The chart is advantageous in the following situations: [3] The sample size is relatively small (say, n ≤ 10— ¯ and s charts are typically used for larger sample sizes) The sample size is constant; Humans must perform the calculations for the chart
The process capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., C pk or C pm) or as a process performance index (e.g., P pk or P pm).
The chart is advantageous in the following situations: [3] The sample size is relatively large (say, n > 10— ¯ and R charts are typically used for smaller sample sizes) The sample size is variable; Computers can be used to ease the burden of calculation
The table shown on the right can be used in a two-sample t-test to estimate the sample sizes of an experimental group and a control group that are of equal size, that is, the total number of individuals in the trial is twice that of the number given, and the desired significance level is 0.05. [4]
Simple example of a process control chart, tracking the etch (removal) rate of Silicon in an ICP Plasma Etcher at a microelectronics waferfab. [1] Time-series data shows the mean value and ±5% bars. A more sophisticated SPC chart may include "control limit" & "spec limit" % lines to indicate whether/what action should be taken.
Steps for using sample size tables: Postulate the effect size of interest, α, and β. Check sample size table [20] Select the table corresponding to the selected α; Locate the row corresponding to the desired power; Locate the column corresponding to the estimated effect size. The intersection of the column and row is the minimum sample size ...