Using the ODD protocol and NetLogo to replicate agent-based models

Replicating existing models and their key results not only adds credibility to the original work, it also allows modellers to start model development from an existing approach rather than from scratch. New theory can then be developed by changing the assumptions or scenarios tested, or by carrying out more in-depth analysis of the model. However, model replication can be challenging if the original model description is incomplete or ambiguous. Here we show that the use of standards can facilitate and speed up replication: the ODD protocol for describing models, and NetLogo, an easy-to-learn but powerful software platform and language for implementing agent-based models. To demonstrate the benefits of this approach, we conducted a replication experiment on 18 agent-based models from different disciplines. The researchers doing the replications had no or little previous experience using ODD and NetLogo. Their task was to rewrite the original model description using ODD, implement the model in NetLogo and try to replicate at least one exemplary main result. They were also asked to produce, if time allowed, some initial new results with the replicated model, and to record the total time spent on the replication exercise. Replication was successful for 15 out of 18 models. The time taken varied between 2 and 12 days, with an average of 5 days. ODD helped to systematically scan the original model description, while NetLogo proved easy and quick to learn, but difficult to debug when implementation problems arose. Although most of the models replicated were relatively simple, we conclude that even for more complex models it can be useful to use ODD and NetLogo for replication, at least for developing a prototype to help decide how to proceed with the replicated model. Overall, the use of both, standard approaches such as ODD and easy to learn but powerful software such as NetLogo, can promote coherence and efficiency within and between different models and modelling communities. Imagine if all modellers spoke ODD and NetLogo as a common language or lingua franca.