Defining Variables for EWMA X-bar and R/S Chart - Quick Tab

Select the Quick tab of the Defining Variables for Variability Chart dialog box to access the options described here.

Note: Analyzing Raw Data vs. Aggregated Data

With Statistica, you can create X-bar and R or S charts from raw data as well as aggregated data. In many quality control applications, the measurement devices or data bases that collect the measurements for critical variables will automatically aggregate the raw data, and only report (store) the sample means and ranges (or standard deviations). The options for analyzing aggregated data are only available for variable control charts of means (X-bar charts), and not for charts of individual observations (where this option is not applicable).

Input is raw data (Compute means, etc.)

Select this option button if the input data consist of raw measurements; for X-bar and R or S charts, the program will then compute the sample means and ranges or standard deviations from the data, and plot those statistics in the requested quality control charts.

Input is aggregated data (Data contains means, etc.)

Select this option button if the input file already contains aggregated summary statistics,(sample means and ranges or standard deviations). In this case, in the results dialog box, various options for charts and other results tables that require access to the raw data will not be available. On the other hand, if for your QC application you are already storing the respective summary statistics in a database (which you are accessing via this module), then creating charts from aggregated data will be more efficient and may be faster.

Variables

Click the Variables button to display a standard variable selection dialog box. From here, you can select a variable with measurements, a variable with sample identifiers (optional), and a Variable with part identifiers (to specify a short run chart).

Constant sample size

Select this check box if you do not have a variable with sample identifiers. Specify the sample size for variable control charts by typing the number in the box or by adjusting the number with the microscrolls.

Constant number of samples per part

 If this check box is selected and no part identifier variable is selected, you can specify here a fixed number of consecutive samples for each part. For example, if your input data consist of batches with different specifications (i.e., different parts) from which a fixed number of samples is automatically drawn, then you can select this option so that on reading the data each chunk of k consecutive samples is automatically assigned to the same part.

Minimum number of observations per sample

 Specify a minimum n per sample by typing it in or by adjusting the number with the microscrolls. Samples with fewer observations than the minimum specified will be ignored.

Lambda for exponentially weighted moving average

Specify here the value of lambda for constructing the exponentially weighted moving average line (chart).
 See also, computational details for more information.