Having a goal in mind when generating this hypothesis is critical. You should write down in a notebook what the goal is and what metric you want to test. Defining the metric is also critical at the outset since it easy to stray from it. For example, If your testing a new ad type, then your hypothesis should probably be written in terms of CTR and not CPA. Your results might show a better CPA, but this should not sway your decision since your hypothesis is framed in terms of CTR and you should not apply the experiment!