Weibull Analysis: Results (Raw Data) - Quick Tab

Use the Variable << and >> buttons (or use the adjacent menu) to select a current variable for which to compute results. These buttons are only available when multiple variables with failure times are selected on the Weibull & Reliability/Failure Time Analysis dialog

- Raw Data

tab.

Use the boxes in the Current parameter values/estimates group to change the parameters for the Weibull distribution. By default, the values in the Offset (threshold/location), Shape parameter, and Scale parameter fields will all show maximum likelihood parameter estimates for the current data for the three-parameter Weibull distribution (unless the parameter estimation procedure failed to converge). See the Introductory Overview for additional details concerning the three-parameter Weibull distribution.

Enter a value in the Offset (threshold/location) box (or use the accompanying microscrolls) if you want to change the offset (threshold/location) parameter for the Weibull distribution. This box is only available if the ML shape and scale parameters option button or the User-defined parameters options button is selected via the Parameter values/estimates group box on the Advanced tab.

Enter a value in the Shape parameter box (or use the accompanying microscrolls) if you want to change the shape parameter for the Weibull distribution. This box is only available if the User-defined parameters options button is selected via the Parameter values/estimates group box on the Advanced tab.

Enter a value in the Scale parameter box (or use the accompanying microscrolls) if you want to change the scale parameter for the Weibull distribution. This box is only available if the User-defined parameters options button is selected via the Parameter values/estimates group box on the Advanced tab.

Click the Summary: Parameters button to display a spreadsheet with the current parameter values. If the current parameter values are the maximum likelihood estimates (the default values were not changed in the Shape parameter or Scale parameter boxes, see above), then the spreadsheet will also contain the standard errors and confidence intervals for the parameter estimates. Note that the confidence intervals are based on the information matrix from the maximum likelihood estimation procedure, and they are only approximations (based on the normal distribution). In addition, in the case of the shape and scale parameters, they are adjusted to reflect the fact that the valid parameter space is bounded (i.e., shape and scale must be greater than 0). See Nelson (1990; Section 5.7) or Dodson (1994) for computational details. Also, Dodson (1994) cautions against the interpretation of confidence intervals computed from maximum likelihood estimates when the shape parameter is less than 2; in that case the variance estimates computed for maximum likelihood estimates lack accuracy.

If the values were changed in the Shape parameter or Scale parameter boxes (see above), then the spreadsheet will contain only the current parameter values.

The percentiles for the confidence limits for the parameter estimates (see above) depend on the value entered into the CL (Confidence limit) box. By default Statistica will compute 95% confidence intervals. Note that the (percentile) value entered here will also be used to construct the confidence intervals for the various results spreadsheets and graphs involving the reliability and  distribution functions (see Weibull Analysis: Results (Raw Data) - Reliability and distribution tab).

Click the Recompute button in order for Statistica to "read" the current value of the Offset (threshold/location) parameter, and then compute maximum likelihood parameter estimates for the Shape and Scale parameters based on the respective Offset (threshold/location) parameter. Note that this button is only available when you select the ML shape & scale params option button on the Weibull Analysis: Results (Raw Data) - Advanced tab.

See the Introductory Overview section for a summary description and discussion of these options.