The FDA does not actually approve a phase 3 (or any phase) study design. They offer guidance, Companies are wise to listen to the guidance and incorporate it into design but the ultimate outline or details of the protocol is not approved by the FDA.
Drug developers are free to ask for help from FDA at any point in the drug development process, including:
Pre-IND application, to review FDA guidance documents and get answers to questions that may help enhance their research
After Phase 2, to obtain guidance on the design of large Phase 3 studies
Any time during the process, to obtain an assessment of the IND application
Even though FDA offers extensive technical assistance, drug developers are not required to take FDA’s suggestions. As long as clinical trials are thoughtfully designed, reflect what developers know about a product, safeguard participants, and otherwise meet Federal standards, FDA allows wide latitude in clinical trial design.
If the details of the SAVA data, not yet presented, shows that 20+ performs much better than 16-20 then they will narrow the range in phase 3. Otherwise they may make it 16+ or have one criteria in one study and another in the second study.
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