  Many evaluation methods have been applied to assess the usefulness of visual
analytics solutions. These methods are branching from a variety of origins with
different assumptions, and goals. We provide a high-level overview of the
process employed in each method using the generic evaluation model "GEM" that
generalizes the process of usefulness evaluation. The model treats evaluation
methods as processes that generate evidence of usefulness as output. Our model
serves three purposes: It educate new VA practitioners about the heterogeneous
evaluation practices in the field, it highlights potential risks in the process
of evaluation which reduces their validity and It provide a guideline to elect
suitable evaluation method.
