To be considered evidence, the assumption/hypothesized relationship that you are seeking evidence for must have been measured or observed. Opinions that something should work, even when contained in a peer reviewed paper, do not count. For example, if sustainability standards are expected to change corporate behavior, then examples where sustainability standards were followed by widespread adoption across the industry would be required to constitute evidence. All knowledge sources have the potential to hold evidence for our assumptions — not just peer reviewed literature. The practitioner’s job is to discern whether a quantitative or qualitative assessment from any source deals with an observation of the assumption being true or false (evidence) or states its potential (not evidence).