# Probit Analysis Options

You can specify options for your probit analysis:

**Statistics**. Allows you to request the following optional statistics: Frequencies,
Relative median potency, Parallelism test, and Fiducial confidence
intervals.

- Relative Median Potency. Displays the ratio of median potencies for each pair of factor levels. Also shows 95% confidence limits for each relative median potency. Relative median potencies are not available if you do not have a factor variable or if you have more than one covariate.
- Parallelism Test. A test of the hypothesis that all factor levels have a common slope.
- Fiducial Confidence Intervals. Confidence intervals for the dosage of agent required to produce a certain probability of response.

Fiducial confidence intervals and Relative median potency are unavailable if you have selected more than one covariate. Relative median potency and Parallelism test are available only if you have selected a factor variable.

**Natural Response
Rate.** Allows you to indicate a natural response rate even
in the absence of the stimulus. Available alternatives are None, Calculate
from data, or Value.

- Calculate from Data. Estimate the natural response rate from the sample data. Your data should contain a case representing the control level, for which the value of the covariates is 0. Probit estimates the natural response rate using the proportion of responses for the control level as an initial value.
- Value. Sets the natural response rate in the model (select this item when you know the natural response rate in advance). Enter the natural response proportion (the proportion must be less than 1). For example, if the response occurs 10% of the time when the stimulus is 0, enter 0.10.

**Criteria.** Allows you to control parameters of the iterative parameter-estimation
algorithm. You can override the defaults for Maximum iterations, Step
limit, and Optimality tolerance.

Specifying Probit Analysis Options

This feature requires SPSS® Statistics Standard Edition or the Regression Option.

- From the menus choose:
- In the Probit Analysis dialog box, select Options.