Harry Brignull at 90% of Everything:
It’s important to always state observations separately from your design recommendations, and to try to state them in a pair. Observations are facts like “Participant 3 failed to create an account successfully” or “There’s a 25% drop out rate on step 3 of the wizard.” If you start with these facts, you add structure to the discussion: all the stakeholders can do at this point is acknowledge that there is a problem, or discuss the validity and generalisability of the observation. Once you’ve cleared this hurdle, then you can move onto the fun bit: design recommendations.
One common mistake is to make a blanket statement about user research, followed by a design recommendation e.g. “User research indicated that there were problems with this area, so we should change all the labels as follows…” This is bad because it’s opaque. What user research? What problems exactly? Without establishing a bedrock of fact, your recommendations cannot be evaluated properly and could lead you into a bad place.
Before entering into a debate you prepare evidence which supports your position. As UX designers we need to do the same preparations before making design recommendations. Disclosing the facts always makes for a stronger recommendation.