Analysis groups provide negative values for the group at the top level of the system model

Question

Hi,

I am trying to use the analysis group function for contribution analysis of my system. The problem is, I am often getting negative impacts (for multiple impact categories) in the group that is assigned to my main process at the top level. This group is supposed to represent direct emissions, so all direct elementary flows should be assigned to it and all other input processes and their providers are assigned to different groups. Even though my direct elementary flows should have a positive impact (even confirmed by "direct impacts" tab), the group has negative impacts. Also when looking at the contribution tree results, I cannot locate any negative contributions to the impacts, only positive

I am afraid the problem might be of some aggregation of processes with the same name in the supply chain, which might lead to double counting and these double counted impacts are then subtracted from the total impacts of the group at the top level – thus leading to negative numbers. This is only a speculation though. 

Does anybody have any experience with this or some tips how to solve it? Thank you!

Answer 1

Indeed, if your system and the analysis groups are very complicated, this can lead to unexpected results, as the underlying algorithm is also somewhat complicated. Make it simpler, then, which is typically possible. To give more advice I would need more details, so either post here or ask in Nexus for a private modeler support. Thank you!

Answer 2

Thank you Andreas!

To anyone interested, I managed to solve the issue by splitting my complex process into multiple subprocesses for each group that I wanted to analyse. Then I combined these subprocesses into one process with each process being assigned a group in the product system model scheme.

Not the most elegant way to solve the issue, as it requires way more processes now to be created, but it works.