Everyone who interviews will get a decision of acceptance, alternate list, or rejection. I think the average time for a decision is 4-6 weeks but it can depend on the cycle. I can't remember the logistics behind the Oct 15 rule but if you interviewed 6-7 weeks ago and haven't heard back, I would call after Oct 15 if you still haven't gotten a decision. Generally, if it has been more than 6-7 weeks since you interviewed, I would call the office and just let them know that you haven't heard back and would like to know when to expect a decision. Also, keep in mind that SDN is a very tiny portion of the entire applicant pool--there are plenty of lurkers and people are also less likely to post about rejections.
Each third of the waitlist has over 100 candidates so it is not unusual for people in the top third to not get in--I had interviewed on the same day as another SDNer and he was placed in the top third and I was in the middle third. I ended up getting accepted and he never did, and it really just depends on the cycle and what kind of applicants give up their seats since people are taken off the waitlist to best match the candidate who withdrew their acceptance. "Matching" can include IS/OOS, ethnicity, gender, stats, extracurriculars, unique experiences, etc. SO for example, if a OOS non-white ORM male candidate who has lots of experience in global health/outreach gives up their seat, then the adcoms will try to find someone who is similar to that in order to keep the class as diverse as possible. People in the top third are considered first BUT if they cannot find a similar candidate, they will move to the middle third then to the bottom third. THEY DO NOT ACCEPT EVERYONE OFF THE TOP THIRD BEFORE MOVING TO THE MIDDLE THIRD so while the top third has the "best chance", if no one has a similar profile they will move down the list. Also, once you are placed in a specific third, you stay there for the duration of the cycle--there is no such thing as moving up/down the waitlist. Hope this helps!
