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Statit Custom QC & Quality Improvement

Variable Charts

X-Bar Chart
An X-bar chart is used to control the process mean. Each point on the chart represents the average of a subgroup.

Subgroups of 2 to 30 samples may be used when computing the control limits for the X-bar chart when based on the range.

Optionally, there is no limit to the subgroup size when limits are based on sigma. If the subgroup size varies, the computed limits will also vary accordingly.

R Chart
The R chart is used to control the process variability. Each point on the chart represents the range of a subgroup. Subgroup sizes may be as small as 2 or as large as 30.

s Chart
The s chart is also used to control the process variability. Each point on the chart represents the standard deviation of a subgroup. There is no limit to the subgroup size.

Median Chart
The median chart is used to control the centering of the process. Each point on the chart represents the median of a subgroup.

The limits are based on the median range and may optionally be changed to use the average range.

The center line may be computed based on the average or median of subgroup medians. Subgroups may be as large as 25 samples.

Individual Chart
An individual chart displays each value. A subgroup size is used to compute the limits, with value of 2 being most common, although the subgroup size may be as large as 30.

Moving Range Chart
The moving range chart is used to control variability of processes which do not form natural subgroups.

Moving Average Chart
Either uniformly weighted or exponentially weighted moving average (EWMA) charts may be generated.

Cumulative Sum Chart
This chart may be used to monitor a two-sided, variable process. The subgroup size may be as large as 30 samples.

Two tests for out-of-control indication are available: Run Sum and V-mask. The Run Sum may either be reset to zero or to a fast initial response (FIR) value when an out-of-control value is reached. Optionally, a V-mask may be specified. A cumulative sum chart for attribute data is also included.

Process Capability Analysis
This graph superimposes a normal curve over a histogram of the data and displays numerous values such as Cp, Pp, Cr, Cpk, Ppk, CPU, CPL, both actual and estimated percent beyond specification, etc. This may be used for data collected in subgroups or individually.

Attribute Charts

p Chart
The p chart shows either the fraction or percent of nonconforming units in each sample. The sample size may vary.

np Chart
The np chart shows the number of nonconforming units in each sample.

c Chart
The c chart shows the number of nonconformities per inspection unit.

u Chart
The u chart shows the number of nonconformities per inspection unit when the inspection unit changes.

Pareto Chart
The Pareto chart displays a bar chart of discrete characteristics with their cumulative percentage of total superimposed. The number of characteristics is limited only by the resolution of the display device.

Optionally, if a maximum number of values is specified, all remaining characteristics will be automatically included in an OTHER category.

Cumulative Sum Chart
The cumulative sum chart is used for detecting an increase or decrease in the number of nonconformities per unit from a known target value.

Additional Features

Most of the variable and attribute charts offer these features:

Data Table
The Table feature presents a list of items to display below the chart. The choices include the underlying data, plotted values, control limits, etc. Other variables may also be specified.

Data Tips
When the chart is viewed within Statit and a Data Tip is clicked on, then a pop-up window is opened, displaying additional information. Data Tips are defined for the curve markers, which display, by default, the underlying data, plotted value and X-axis identification. Other items may also be displayed, such as the control limits and other user-specified variables. Other Data Tips available include "Summary", which displays the number of points on the chart, the subgroup size, the number of sigma control limits, etc.; "Rules Tested" displays all the rules which were tested; and others. Custom Data Tips may also be easily added to the chart.

Data Highlighting
When data highlighting is requested, then a curve marker may be clicked-on. This results in the underlying data for the point selected being highlighted in the Data Window.

Web Settings
Developers may define Web pages/scripts which will be automatically loaded when a data marker or out-of-control letter is clicked-on when the chart is viewed in a Web browser.

This feature is used in conjunction with a categorical variable. When the value of the categorical variable changes, then the new value is displayed above the data space in a block-style legend. This may be helpful in identifying special causes of variation or trends. Also, Statit may be instructed to recalculate the control/warning limits within each category defined by a categorical variable.

Automatic Alarms
When rules testing reveals an out-of-control situation, then several pre-defined alarms may be generated. These include changing the background color of the chart and displaying a text message. As well, a chart can be generated, yet only displayed if rule violations are detected.

Rules Testing
AT&T/Western Electric rules testing is fully implemented. Additionally, Statit offers the ability to add new rules or change the existing ones.

Points which violate a rule are indicated by a letter near the point on the chart and the description of the rule is displayed either as a Tip or at the bottom of the graph.

Assignable Cause
Points on the chart may be excluded from control/warning limit calculations and rules testing. This feature is often used to identify out-of-control points, for which an assignable cause has been found, with a text string. The string is displayed on the chart by the point.

Text may be placed by any point on the chart. This text will automatically follow the point as it moves across the chart when more data is collected. This, for example, allows an operator to identify the point as adjustments are made to the process.

Control Limits
The control limits may be either calculated or user-specified. If calculated, the coefficient of sigma may be easily changed to something other than the default of 3-sigma. Either of the limits may be turned off.

Warning Limits
Warning limits may be requested, and are placed by default at plus and minus 2-sigma. Either limit may be turned off.

Specification Limits
Upper, center and/or lower specification lines may be displayed.

Subgroup Identification
Subgroups may be identified either by the constant subgroup size or by a categorical variable, such as date/time. The horizontal axis may be numeric or character. This permits subgroups to be identified by date, time, lot number, etc. The size and degree of rotation of the tick mark labels may also be specified.

Multiple Charts
The QC charts, and all other graphics furnished with Statit, may be sized and positioned anywhere on the graph. This permits the design of custom graphs easily and quickly.

Text Reports
A text report may be requested which lists the control limits, rules tested and violated, and outliers. For points which violate a rule, or are outliers, the raw data is listed along with the X-axis identification.

Fishbone Diagrams
Fishbone charting can be used to display the names and relationships of factors which contribute to a particular problem.

Design Of Experiments
Production processes are affected by many factors that contribute to the variation in quality. Statit’s Design of Experiments procedures may be used to find the optimum settings of the various factors to yield the best results.

Statit can be used to design and analyze factorial experiments with up to eleven factors, each at two different settings. The specified number of runs may result in full or fractional designs. Some available features include:

The factor names and levels may be user-specified.
Runs are automatically randomized; blocking may optionally be requested.
The maximum order of the model may be specified to remove the high order aliasing.

For central composite or Box-Behnken designs, Statit can generate the design and then analyze the data collected. Up to six factors may be examined. The results consist of parameter estimates, an ANOVA table, contour plots, and normal probability plot of residuals.

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