Bug Fixed: User-defined equations containing fractions of same-base exponents evaluated incorrectly

Last modified July 9, 2021

The big picture

With the release of Prism 9.2.0, an issue was fixed with the way some user-defined equations were simplified for evaluation. Prior to this fix, some user-defined equations containing fractions of exponents with the same base were simplified incorrectly, resulting in missing results. This issue affected earlier versions of Prism 9 (but not Prism 8), and has been corrected in Prism 9.2.0. However, the curves generated as a result of this issue would have been almost immediately wrong. It's unlikely that you would have experienced this issue without noticing it. That said, these user-defined equations should once again work as expected. Additionally, this issue had no effect on any of the existing built-in equations offered in Prism's equation library, and we've added additional test cases to ensure that similar issues will be detected in the future.

The details

Prism's equation evaluation engine includes a set of rules that it uses to simplify user-defined equations based on basic properties of mathematical operations and functions. For example:

The issue that was fixed in Prism 9.2.0 involved updating the following incorrect rule definition:

to the mathematically correct definition:

Note that only user-defined equations that utilized this form of same-base exponent fraction would have been affected. If the user-defined equation included the simplified a^(b-c) version initially, Prism would have properly evaluated this expression in both 9.2.0 and previous versions.

Explore the Knowledgebase

Analyze, graph and present your scientific work easily with GraphPad Prism. No coding required.