The four classifications used in typology of evidence
| A | B | C | D | |
|---|---|---|---|---|
| What works: elements of an intervention that work (intervention demonstrated to work in evaluation research) | What works is Known | What works is Known | What works is Known | What works is Known |
| How it works: cause-effect interaction among those elements (the components and how they work) | How it works is Known | How it works is Known | How it works is Not known | How it works is Not known |
| Repeatability | Repeatability is Universal | Repeatability is Limited | Repeatability is Universal | Repeatability is Limited |