More info on common lottery algorithm

[Anonymous]

New in the Q&A for the common lottery. Sounds just like the process we ultimately came to in the other thread on the common lottery. (Thought I would start a new thread, since that one was so long.) Notably, there is nothing here about a process similar to New Orleans, where there are trades made so that if I get into A but ranked B higher and you get into B but ranked A higher, we switch and both get into our top choice school.

Here is the info:

http://www.myschooldc.org/faq/#common-14

How does the lottery matching algorithm work?

The My School DC lottery is designed to match students with the schools they want most, and maximize the number of students who are matched. The matching algorithm is the computer program that runs the lottery. It is a deferred acceptance model that was developed specifically for My School DC by the Institute for Innovation in Public School Choice (IIPSC) and is based on the Nobel Prize-winning work of economist Al Roth of Stanford University. IIPSC also developed the lottery algorithms for New Orleans and Denver and is currently working with Philadelphia and Newark.

The two most important things to know about the algorithm are:

Because of the way the algorithm works, students who rank schools on their application according to their true choices have the best chance of being matched to the schools they most want to attend.

Students who apply early get no advantage in the matching process.

For all schools except DCPS specialized high schools, the algorithm follows this process.

The matching algorithm assigns each student a random lottery number and attempts to match each student with his or her first choice first, then his or her second choice, and so on. A student’s ranking of schools (the order a student puts schools on his or her application) is critical because the matching algorithm attempts to match each student with his or her first choice first, then his or her second choice, and so on. However, when the matching algorithm is comparing two students who have applied to the same school, the decision is based on two criteria: (1) each student’s preferences at that school (e.g., sibling preference); and (2) if there is no difference in preferences, each student’s randomly assigned lottery number. The students’ rankings of the schools are not a factor at that point in the process. This is why the system is strategy-proof — and why students are best served by ranking schools according to their true choices (the school they most want to attend first, then their second choice, and so on).

Each student is waitlisted at every school he or she ranked higher than the school to which they were matched. (It is possible for a student to be waitlisted at every school on his or her list if they are not matched.) Siblings who apply to the same schools also are waitlisted at any school where one of their siblings is matched.

The six DCPS specialized high schools admit students based on specific criteria. Each of the specialized high schools identifies which applicants meet their specific minimum requirements, and then each school determines the order of priority among that group. The matching algorithm uses the process described above: It attempts to match each student with his or her first choice first, then his or her second choice, and so on. (The student’s ranking may include any combination of DCPS schools, public charter schools, and DCPS specialized high schools.) However, when the matching algorithm is comparing two students who have applied to the same specialized high school, the decision is made based on the school’s order of priority for the applicants.

[Anonymous]

So the way I read that is that, without any preferences, it comes down to lottery #. If you get a low (early) one, you have a good chance of getting in to your top school or schools. If you have a high (late) one, your chances aren't looking so hot for your top schools.

With any preferences (IB or sibling or OOB with sibling) your chances are better for your top school, irregardless of your lottery #.

Is this correct?

[Anonymous]

Well, except it's not that they take your #, go to your first, second, third, etc. I think it's the bidding process described on the last thread, so everyone makes a request of their first choice, and based on the lottery order each school has they accept/deny, but it's only temporary. Then people not assigned make a request to their next-ranked school, etc. It's the student-initiated deferred acceptance algorithm. I think it's too bad if they don't use the New Orleans additional feature where trades are made. Seems like that would lead to mutually-beneficial outcomes.

[Anonymous]

It states "the matching algorithm assigns each student a random lottery number" so early submission does not equal a low number.

[Anonymous]

Anonymous wrote:So the way I read that is that, without any preferences, it comes down to lottery #. If you get a low (early) one, you have a good chance of getting in to your top school or schools. If you have a high (late) one, your chances aren't looking so hot for your top schools.

With any preferences (IB or sibling or OOB with sibling) your chances are better for your top school, irregardless of your lottery #.

Is this correct?

Saying "top school" confuses things.

If you have a preference, you are only competing against people of the same or higher preference. It's impossible for a higher-preference student to lose out to a lower-preference student. The only person you can lose to is a higher preference, or an equivalent preference with a better lottery number. It's not so much that your chances are better, but rather that you're in a different lottery altogether.

The key is, rank your choices in order of preference, do not use your perception of the odds of being successful in your ranking. Once you get into one school, you get waitlisted at your higher picks but not at your lower picks.

[Anonymous]

Anonymous wrote:Well, except it's not that they take your #, go to your first, second, third, etc. I think it's the bidding process described on the last thread, so everyone makes a request of their first choice, and based on the lottery order each school has they accept/deny, but it's only temporary. Then people not assigned make a request to their next-ranked school, etc. It's the student-initiated deferred acceptance algorithm. I think it's too bad if they don't use the New Orleans additional feature where trades are made. Seems like that would lead to mutually-beneficial outcomes.

The way the DC system is structured mutually beneficial trades are impossible. This is because in effect they do take each bidder and go first, second, third, until a seat is found. At the same priority level, a person with a better lottery number will never lose a seat to a person with a worse lottery number because of their relative rankings. A lower-ranked person can never have what a higher-ranked person wants, so trades are impossible.

[Anonymous]

So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

[Anonymous]

Anonymous wrote:So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

It sounds like 1 number. Not 12 different numbers.
So it essentially boils down to your lottery number, it seems.

[Anonymous]

Anonymous wrote:

Anonymous wrote:So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

It sounds like 1 number. Not 12 different numbers.
So it essentially boils down to your lottery number, it seems.

Wow, no words for how much that sucks for folks with really bad numbers. Like, no words. That's why I wish wish wish how the student ranks the school DID come into play. If I'm in the same preference category (let's say no preference at all - no sib pref, no IB pref), and I (Suzie) ranked school A #2, and Johnny ranked school A #5 but didn't get into 1-4 yet (and he also has no preferences), if his lottery number is better than mine he's going to get the spot. That SUCKS. I think the student who wanted it badly enough to put it at 2 should get it over the person who put it at 5, if all else is equal except lottery number. Yuck, so much for true 2-way matching.

[Anonymous]

What a colossal bummer. As someone who majorly struck out on the lottery last year, I fail to see how this improves the winners and losers situation that's prevailed in past years, other than making it occur on a larger scale. Some kids stand to win big by having great odds at getting into their top choice, or even one of their top choices, and some kids have awful odds of getting into anything.

[Anonymous]

Anonymous wrote:

Anonymous wrote:Well, except it's not that they take your #, go to your first, second, third, etc. I think it's the bidding process described on the last thread, so everyone makes a request of their first choice, and based on the lottery order each school has they accept/deny, but it's only temporary. Then people not assigned make a request to their next-ranked school, etc. It's the student-initiated deferred acceptance algorithm. I think it's too bad if they don't use the New Orleans additional feature where trades are made. Seems like that would lead to mutually-beneficial outcomes.

The way the DC system is structured mutually beneficial trades are impossible. This is because in effect they do take each bidder and go first, second, third, until a seat is found. At the same priority level, a person with a better lottery number will never lose a seat to a person with a worse lottery number because of their relative rankings. A lower-ranked person can never have what a higher-ranked person wants, so trades are impossible.

No, that is incorrect. That is not how the deferred acceptance algorithm works. Essentially, what matters most is preference (IB, Sibling, etc.) and then within those preferences it is random assignment by lottery number. So it could be that I get into school B and you get into school A, but I've ranked A higher than B and you've ranked B higher than A, and a mutually beneficial trade would be possible. That's why New Orleans after running the deferred acceptance algorithm then tries to make trades to get to the best outcome possible for everyone. It does not sound like DC is doing this last part of the model, and that's too bad.

[Anonymous]

Anonymous wrote:So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

Your lottery number is not the same for all schools, so your second scenario is not accurate. No worries there. But it's not exactly as simple as each school running a separate lottery, because rather than having the schools run the lottery and then choose the students, the students in the algorithm do the requesting of the spot. They are temporarily assigned and reassigned in many rounds. In the end, your relative waitlist number will vary in the schools you apply to because their "rank" of you depends first on preference (they will always choose someone who is IB over someone who is OOB, e.g.) and then, within that preference band, by random lottery number.

As PP said, all you need to do is list in the order of preference.

[Anonymous]

Anonymous wrote:

Anonymous wrote:So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

It sounds like 1 number. Not 12 different numbers.
So it essentially boils down to your lottery number, it seems.

I agree that the text of the FAQ implies a single lottery number for all schools. In practice not only does that not seem fair, but it seems almost impossible. Each school has different pools of students in each preference group. So a single "lottery number" for the entire system doesn't make a lot of sense. I am 95% sure that they will clarify that each student is randomly ordered within the appropriate preference group of each school. Nothing else makes any sense.

[Anonymous]

Anonymous wrote:

Anonymous wrote:So do you get one, single randomly assigned lottery number that applies to all 12 of your selections? Or are you entered essentially into almost 12 separate lotteries for each school, with a random assigned number for that school?

Man would it SUCK to get a crappy number for all 12 choices. Wouldn't that essentially (unless you have both IB and sibling preference for your 1st choice or something) knock you out of the whole lottery because, at least for the more popular schools, there is guaranteed to be more people with better numbers who ranked it high enough to be competing with you?

Your lottery number is not the same for all schools, so your second scenario is not accurate. No worries there. But it's not exactly as simple as each school running a separate lottery, because rather than having the schools run the lottery and then choose the students, the students in the algorithm do the requesting of the spot. They are temporarily assigned and reassigned in many rounds. In the end, your relative waitlist number will vary in the schools you apply to because their "rank" of you depends first on preference (they will always choose someone who is IB over someone who is OOB, e.g.) and then, within that preference band, by random lottery number.

As PP said, all you need to do is list in the order of preference.

Sorry, should have been more clear - your lottery number ORDER is not the same for all schools. Meaning, within preference catgories, each school then randomly chooses an order for the people who have applied. So you could be without a sibling at TR and have a relatively good spot on the waitlist and without a sibling at MV and be in a terrible waitlist position.

[Anonymous]

Anonymous wrote:What a colossal bummer. As someone who majorly struck out on the lottery last year, I fail to see how this improves the winners and losers situation that's prevailed in past years, other than making it occur on a larger scale. Some kids stand to win big by having great odds at getting into their top choice, or even one of their top choices, and some kids have awful odds of getting into anything.

I had this conversation with people from the Common Lottery helpline a month ago - I brought up that I noticed last year, because you could click on your DCPS desired school's admit and waitlist and then click on the random assigned numbers of the people ahead of you to see hwo they'd ranked their schools - and when I looked at most of the people who were in single digits, they were on single digits on ALL or MOST of their waitlists. Whereas many people who were #320 on one had sucky sucky 200s-300s numbers for all their choices too. It wasn't true 100%, but it was true enough at the many random people I looked at, and it was true of us (but we were lucky to have our numbers all be in the teens and 20s). The lottery staff said "No no that won't happen again this year", but really, it will, because preference people had preference before, and they do now, and there will be a computer attempt to go down each person's desired rankings, but at the end of the day the computer will use random lottery number, NOT desired school ranking, to choose between people who otherwise everything else is equal. With the exception of the number of people who previously would have applied to 20 charters and now have to rank most of them along with DCPS (and the double/triple spot-holding that that leads to), it really doesn't sound like it's that much different in terms of giving you a better chance at YOUR #1 ranked school. But my understanding from the school lottery folks was that you DID have a better chance under this model.