Prioritization does not require certainty. You can make a reasonable hypothesis based on the objective, likely magnitude, controllability, or the information you already have.

Explain the reason for the priority out loud. For example, start with the branch that could have the largest impact on the objective or the one most likely to distinguish between competing explanations.

If new data disproves the priority, change it. That shows structured judgment rather than guessing. If useful, this guide on case structure vs. case framework gives more detail.