early as 2003. Implementation Many ABM frameworks are designed for serial von-Neumann computer architectures, limiting the speed and scalability of implemented models. Since emergent behavior in large-scale ABMs is dependent of population size, scalability restrictions may hinder model validation. Such limitations have mainly been addressed using distributed computing, with frameworks such as Repast HPC specifically dedicated to these type of implementations. While such approaches map well to cluster and supercomputer architectures, issues related to communication and synchronization, as well as deployment complexity, remain potential obstacles for their widespread adoption. A recent development is the use of data-parallel algorithms on Graphics Processing Units GPUs for ABM simulation. The extreme memory bandwidth combined with the sheer number crunching power of multi-processor GPUs has enabled simulation of millions of agents at tens of frames per second. Integration with other modeling forms Since Agent-Based Modeling is more of a modeling framework than a particular piece of software or platform, it has often been used in conjunction with other modeling forms. For instance, agent-based models have also been combined with Geographic Information Systems (GIS). This provides a useful combination where the ABM serves as a process model and the GIS system can provide a model of pattern. Similarly, Social Network Analysis (SNA) tools and agent-based models are sometimes integrated, where the ABM is used to simulate the dynamics on the network while the SNA tool models and analyzes the network of interactions. Verification and validation Verification and validation (V&V) of simulation models is extremely important. Verification involves making sure the implemented model matches the conceptual model, whereas validation ensures that the implemented model has some relationship to the real-world. Face validation, sensitivity analysis, calibration, and statistical validation are different aspects of validation. A discrete-event simulation framework approach for the validation of agent-based systems has been proposed. A comprehensive resource on empirical validation of agent-based models can be found here. As an example of V&V technique, consider VOMAS (virtual overlay multi-agent system), a software engineering based approach, where a virtual overlay multi-agent system is developed alongside the agent-based model. Muazi et al. also provide an example of using VOMAS for verification and validation of a forest fire simulation model. Another software engineering method, i.e. Test-Driven Development has been adapted to for agent-based model validation. This approach has another advantage that allows an automatic validation using unit test tools. See also Agent-based computational economics Agent-based model in