Page History

...

The natural survival and natural death rates correspond to the lower and upper asymptotes of the sigmoid curve linking response rates and dose. These By default, these asymptotes are estimated , by default, from observed data. However, their value can also be set via Options ‘Options > Advanced > Preferences Preferences’ (or F12), ; the parameters “d d = ” and “a a = ” corresponding   correspond to “the” the lower asymptote and the difference between the asymptotes, respectively.

Example 1. : We are interested in post-treatment survival rates. We know from experience that 10% of the untreated controls will survive (natural survival) and that 20% will die irrespective of the dose administered (natural mortality). The analysis can be carried out using the set values d = 0.1 and a = 0.7.

Example 2. : Now let us assume that 10% of the untreated controls survive, but that the natural mortality rate has not been clearly established. The analysis can be carried out using the set values d = 0.1 and a = 0.7, as was the case in our first example, but with the “fixed” ‘fixed’ option unchecked for parameter “a” a.

Remark 1: In practice, it is rare that for quantal responses to allow estimation of the asymptotes from the observed data. Knowledge from past experiments is usually necessary to specify the asymptotes explicitly. Also note It should also be noted that, for the analysis of variance and the confidence limits, CombiStats does not take into account the fact that the asymptotes are estimated from the data. They are treated, even after estimation, as known and fixed.

...