Natural response rate
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The regression table includes the natural response rate - the probability that a unit fails without being exposed to any of the stress. This statistic is used in situations with high mortality or high failure rates. For example, you might want to know the probability that a young fish dies without being exposed to a certain pollutant. If the natural response rate is greater than 0, you may want to consider the fact that the stress does not cause all of the deaths in the analysis.

You can choose to estimate the natural response rate from the data, or set the value. You would set the value when you have a historical estimate, or to use as a starting value for the algorithm.