What is the best way to evaluate the performance of my marketing spend?

The primary purpose for evaluating the performance of marketing spend tends to be toward informing decisions around how to reallocate that marketing spend in future periods, in order to improve business performance overall. In this regard, we can look at this problem as a decision support problem space. Historically, marketers have used a range of approaches ranging from marketing mix modeling, to marketing attribution, to controlled experiments, and more. None of these approaches are perfect, but a combination of these approaches combined with a rigorous decision feedback loop (e.g. in the spirit of the Observe, Oriented, Decide, Act loop) can help marketers improve their marketing performance management, and hence their marketing performance, over time.