The concern that SOAP will affect the match was not lost on the NRMP. basically, the NRMP wants to fill most of the slots via the main match. The scramble is so horrible for everyone that it entices programs to interview and rank enough people to avoid it. If SOAP becomes much more managable / pleasant, programs won't be as scared of it and hence MIGHT decide to rank less people.
For example, let's say I have 10 spots to fill in my program. And let's say I'm not the most competitive program in the universe. So, I tend to get 7 people I'm really happy with, and 3 whom I'll take but wish I could get "better". With SOAP, I might be willing to shorten my rank list to only those that I'm really happy with. I know that this would likely lead to me having open spots in SOAP. However, I might get better people in SOAP than in the match -- some candidates applying to competitive residencies fail to match, and they may be "better" than those applying in the match.
Big Red has pointed out an issue with SOAP that hasn't been discussed much. SOAP is a "degenerate match" because it's one sided. Only programs submit Rank Lists (now called Preference Lists). Candidates do not submit lists, but instead get offers. If a candidate gets offers during a round, they can accept one, or let them all go and hope for something better. But there's no going back -- once you decline an offer, you can't get back into that program. Therefore, I predict that no candidate is going to decline all offers in a round.
This changes program ranking strategy. IN the main match, I rank people in the order I want them. I cannot hurt myself by ranking someone highly that I think is unlikely to come to my program. If they don't, I just get the next person on my list. I can't "lose" someone by not ranking them high enough, unless people above them match to me (which is a good thing).
In SOAP, this is not true at all. Let's say I have one spot in SOAP. I interview 10 people. One is a superstar, I think he/she is going to get lots of offers. If I put them first on my preference list, they will get an offer from me and probably (due to their "superstar-ness") multiple other offers. They choose one offer, not me. In the second round, I go down my list -- but, if my #2, #3, etc have already taken other offers (because some other program ranked them higher) then they are gone to me -- they can't trade one offer for another. Once they take an offer, they are out of SOAP. So my next offer goes to the next person on my list who is still available.
This makes preference list management vital in SOAP for programs. I might be better off ranking somewhat weaker candidates higher on my list, because I expect that I would be their "best" offer and they would take it. Another way to look at it is this: In the main match, there is no reason for me to worry where I will be on each individual's rank list. Although I'd love to be ranked highly by all, if not I just get the next person on my list. In SOAP, it's critical for me to know how interested an applicant is. Match rules state I'm not allowed to ask. It will be very, very hard to resist. I don't want to "waste" my high ranks on people who are unlikely to come. Candidates won't want to tell me that they aren't as interested, as they'll want as many early offers as possible.
It will be much better than the scramble in many ways. In others, the NRMP is asking for trouble.
