A simple metric is one that is readily available, uncomplicated and easy to use. Its meaning and implications are clear, and it is empowering, because it can be adopted and easily acted upon. 
An example of a not-so-simple metric is Customer Lifetime Value.
“Simple” is one of the 10 Characteristics of an Ideal Metric designated by the Marketing Accountability Standards Board.
- Accountable Marketing: Linking marketing actions to financial performance, 1st Edition; Eds. Stewart, David W. & Gugel, Craig; Routledge, 2016.
- Universal Marketing Dictionary Project, 2023.