|
  |
|
|
| Contents | Index Limits In the Analysis Options dialog box, the Limits group determines the width of the control limits, warning limits, and confidence limits for capability estimates and linear regression. The values entered here directly influence both Type I (false alarms) and Type II (failing to detect process shifts) errors. Values may be entered as either a sigma Ordinate or as a Confidence level, and a given entry will be converted between the two values as this radio button selection is toggled. Ordinate: A sigma ordinate for a control chart measures the distance from the center line to the control limit in units that are one standard deviation wide for the normal distribution. For example, a control limit ordinate of 3.00 will establish control limits at positive and negative 3 standard deviations when a normal distribution is plotted. Confidence level: provides the distribution percentile included within the limits, with equal tail area provided to each side. For example, a normal distribution with 3 sigma limits would correspond to a .9973 percentile. Note: If a non-normal curve fit is used, limits are defined at the appropriate sigma level corresponding to the percentile of the Normal distribution at the selected ordinate. For example, if control limits are defined at an ordinate of 3.00, the lower control limit for a non-normal curve fit on the Individual-X charts and Capability Analysis will be placed at the .135% percentile of the fitted curve, and the upper control limit at 99.865% percentile of the fitted curve, even though these limits may correspond to -1.6 and +7.2 sigma, respectively, for the non-normal distribution. Note: The phrase Confidence level is perhaps statistically inappropriate in the context of control charting. The label for control limits might be more appropriately called “Percent of distribution contained within control limits, with equal probability of being in the tail areas.” Control Limit Enter either the sigma ordinate or the percentile Confidence level that will be used for all control charts and Scatter Diagrams created. Default value is 3.00 sigma, which places the control limits at plus and minus 3 sigma for a normal distribution. Warning Limit: Enter either the sigma ordinate or the percentile Confidence level that will be used as Warning Limits for all control charts. There is no Default value, as the use of Warning Limits is not recommended due to their increase in Type I errors. Capability Interval: Enter either the sigma ordinate or the percentile Confidence level that will be used for determining the Confidence Interval for all Capability Indices. There is no Default value, although an ordinate of 2.575 (95%) might be recommended. Since Capability indices are based upon calculated estimates of the mean, the variation, and the selected distribution, they are subject to error like all other statistics. This error is largely influenced by subgroup size; as subgroup size increases the statistical error will decrease. |
|
|
