Common Lottery Algorithm

[Anonymous]

To the 2 people who explained the above in simple terms, thank you!!! And no, it wasn't TMI .

To everyone else, I wish you well in this debate. I now have a much better idea of my questions about this, although at the end of the day my rankings are still shaping up the same either way.

::dropping a different mic, exiting out different door from other mic-dropper::

[Anonymous]

Not PP, but to everyone else in the thread:

It doesn't matter how they handle your rankings. You can't game this system. There is equal weight between your desires and the desires of the schools. Rank your schools how you feel best represents your preferences. If that means you prefer the 12 most unattainable schools in the district then so be it.

[Anonymous]

Anonymous wrote:It's quite complicated and quite simple:

This is how Gale/Shapley works from a logical approach wrote:
1) Assign lottery numbers to all students

2) Place students into preference pools

3) Sort students in each pool by lottery number

4) in the first pool, iterate the students in the first preference pool

5) if the school has seat for the student then remove a seat from the school/grade

6) move onto the second pool, etc and so on

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This loops and loops on 4-7 until no more students are asking a school for a seat or until no more students are being accepted. If there are 20,000 students then it can loop up to 20,000 times although I imagine that is a rare occurrence.

Since it's a deferred acceptance model a single student can ask for the same seat multiple times. They might not get the seat in the 1st, 2nd or 15,000th iteration but they could on the 15,001st. And that can have an effect an another student and so on an etc. It's also then entirely possible that on the 18,243rd time they give up that seat to take a better one and the whole thing loops again until the closing arguments are met. I believe what the algorithm does is simply this process by applying a mathematical methodology.

Thank you for this! This explains it very clearly.

[Anonymous]

Anonymous wrote:"i" is the student

"s" is the school

I thought that was clear by stating

Student Ranking Schools (based on true preference):
i1: sB, sC, sA
i2: sA, sB, sC

Schools Ranking Students (based on preference and lottery #):
sA: i1, i2
sB: no seats/preference
sC: no seats/preference

anything else that isn't clear?

err.... all of it? I don't usually have trouble following this kind of thing but your description is incomprehensible.

[Anonymous]

Anonymous wrote:Not PP, but to everyone else in the thread:

It doesn't matter how they handle your rankings. You can't game this system. There is equal weight between your desires and the desires of the schools. Rank your schools how you feel best represents your preferences. If that means you prefer the 12 most unattainable schools in the district then so be it.

This is the bottom line. Your "desires" do matter in the actual results, don't let any of these nuts tell you they don't. How they matter beyond order of your acceptance/waitlists... lots of debate. But don't let anyone tell you it doesn't matter how you rank.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:
Show me where it is clearly stated that random lottery number trumps student's ranking of the school and, where all else is equal except the student's rank of the school, a higher random lottery number trumps student's ranking of school?

It's not clearly stated because there isn't anything at all about rank in the FAQ. Let me ask this back: If rank were so important -- more important than lottery number -- why did they leave it out of the FAQ?

I have no idea why they left it out. I only know what the admissions people at the 2 schools I'm most interested in told me and the person representing the lottery at the fair I went to said. They clearly, unequivically said "Rank matters and a student with everything else the same but with who ranked the school lower would lose to a student who ranked it higher".

For me, that's all I need to know, and I'm acting on that regardless of what anonymous people on the internet say. But I'm interested in the sources people are using to assert so strongly that random lottery number trumps rank.

Very strange. When I asked the Schools DC people at the ward 5 school fair, they did not tell me that. They stressed that it was important to put your true first choice in the #1 slot and that your chances of getting in to each school were independent of each other or rank.

[Anonymous]

Anonymous wrote:To the 2 people who explained the above in simple terms, thank you!!! And no, it wasn't TMI .

To everyone else, I wish you well in this debate. I now have a much better idea of my questions about this, although at the end of the day my rankings are still shaping up the same either way.

Yes, this. As we've been saying all along, rank them in the order that you would like your child to attend. You can skip over all the math and trying to parse the tea leaves and you'll have the same chances as everyone else in your preference category.

[Anonymous]

Anonymous wrote:

Anonymous wrote:"i" is the student

"s" is the school

I thought that was clear by stating

Student Ranking Schools (based on true preference):
i1: sB, sC, sA
i2: sA, sB, sC

Schools Ranking Students (based on preference and lottery #):
sA: i1, i2
sB: no seats/preference
sC: no seats/preference

anything else that isn't clear?

err.... all of it? I don't usually have trouble following this kind of thing but your description is incomprehensible.

There is another example in the thread that uses more language based terms to describe the process.

It's quite complicated and quite simple:

This is how Gale/Shapley works from a logical approach wrote:

1) Assign lottery numbers to all students

2) Place students into preference pools

3) Sort students in each pool by lottery number

4) in the first pool, iterate the students in the first preference pool

5) if the school has seat for the student then remove a seat from the school/grade

6) move onto the second pool, etc and so on

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This loops and loops on 4-7 until no more students are asking a school for a seat or until no more students are being accepted. If there are 20,000 students then it can loop up to 20,000 times although I imagine that is a rare occurrence.

Since it's a deferred acceptance model a single student can ask for the same seat multiple times. They might not get the seat in the 1st, 2nd or 15,000th iteration but they could on the 15,001st. And that can have an effect an another student and so on an etc. It's also then entirely possible that on the 18,243rd time they give up that seat to take a better one and the whole thing loops again until the closing arguments are met. I believe what the algorithm does is simply this process by applying a mathematical methodology.

[Anonymous]

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This is the part where I get confused - can anyone clarify? Why would a previous seat be added back?

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:
Show me where it is clearly stated that random lottery number trumps student's ranking of the school and, where all else is equal except the student's rank of the school, a higher random lottery number trumps student's ranking of school?

It's not clearly stated because there isn't anything at all about rank in the FAQ. Let me ask this back: If rank were so important -- more important than lottery number -- why did they leave it out of the FAQ?

I have no idea why they left it out. I only know what the admissions people at the 2 schools I'm most interested in told me and the person representing the lottery at the fair I went to said. They clearly, unequivically said "Rank matters and a student with everything else the same but with who ranked the school lower would lose to a student who ranked it higher".

For me, that's all I need to know, and I'm acting on that regardless of what anonymous people on the internet say. But I'm interested in the sources people are using to assert so strongly that random lottery number trumps rank.

Very strange. When I asked the Schools DC people at the ward 5 school fair, they did not tell me that. They stressed that it was important to put your true first choice in the #1 slot and that your chances of getting in to each school were independent of each other or rank.

it's right here on the myschooldc.org site
http://www.myschooldc.org/faq/#common-3

How does the My School DC common lottery work?
Student-school matches are based on the number of spaces at each school; sibling, proximity, and other preferences; and each student’s choices. (Through the My School DC common lottery, the six DCPS specialized high schools admit students based on specific criteria.)

When there are more students than spaces at a school, students who have a preference (such as a sibling preference) will be the first to be offered spaces. Then, random selection decides which other students will be offered spaces.

Students will be matched with no more than one school. My School DC will try to match each student with their 1st choice, then their 2nd choice, and so on through the student’s list.

Look at the two bold, italics and underlined sentences above. Random selection (lottery number) is part of the school ranking the student. Your ranking and the schools rankings collide. That's it. Your ranking has nothing to do with any other student in the lottery. Your ranking only determines which slots are still open under your application. If you are selected for your first rank then you're other selections are ignored unless you lose your first ranked solution (this is possible because this lottery is running a "deferred acceptance" model. If you lose your first ranking then your rankings below that are open again for consideration. This is when you can take away another child's first ranking (if you're ranking is #2 and theirs is #1 yet you have a better preference/lottery number you get that seat). This is how this entire algorithm works.

[Anonymous]

Anonymous wrote:

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This is the part where I get confused - can anyone clarify? Why would a previous seat be added back?

Not PP, but as it's working through any particular student, they might be temporarily accepted into one school, but then get accepted into one they ranked higher. In that case, they would be assigned to their higher ranked school, and the seat at the first school would be released. All happening in an instant, of course.

[Anonymous]

Anonymous wrote:

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This is the part where I get confused - can anyone clarify? Why would a previous seat be added back?

In the example above - if you didn't get your #1, #2, or #3 seat you could theoretically be seated in #4. If #1 open up (say a child who previous had it ranked the school at #2 and they got into their #1 later in the cycle) then you could ask for that seat again and be granted it because you are now the preferred student. Technically you lost #4 and gained #1. If you were to lose that #1 later in the algorithm then you could ask for #4 again and most likely get it back. This is called "deferred acceptance" and the trickiest part of the lottery to explain.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:
Show me where it is clearly stated that random lottery number trumps student's ranking of the school and, where all else is equal except the student's rank of the school, a higher random lottery number trumps student's ranking of school?

It's not clearly stated because there isn't anything at all about rank in the FAQ. Let me ask this back: If rank were so important -- more important than lottery number -- why did they leave it out of the FAQ?

I have no idea why they left it out. I only know what the admissions people at the 2 schools I'm most interested in told me and the person representing the lottery at the fair I went to said. They clearly, unequivically said "Rank matters and a student with everything else the same but with who ranked the school lower would lose to a student who ranked it higher".

For me, that's all I need to know, and I'm acting on that regardless of what anonymous people on the internet say. But I'm interested in the sources people are using to assert so strongly that random lottery number trumps rank.

Very strange. When I asked the Schools DC people at the ward 5 school fair, they did not tell me that. They stressed that it was important to put your true first choice in the #1 slot and that your chances of getting in to each school were independent of each other or rank.

Seems like every possibility being discussed here means you should put your true rank order, but if rank has nothing to do with your chances of getting in, why do they want you to rank it? Just to know what order to keep you/drop you as the slots fill up? The answer you got actually doesn't mention lottery number or rank, both of which makes your chances relevant to others (even though you can't control lottery number, which is assigned to you). But if the random lottery number has an impact, then what happens to you is still relevant to what happens to others.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

7) whenever a student gets an acceptance into a higher ranked seat (1 over 2, 3 over 5, etc), the previous seat (from a lower rank) is added back into the pool of seats for that school/grade (this is where the student/parent school rankings come into play)

This is the part where I get confused - can anyone clarify? Why would a previous seat be added back?

In the example above - if you didn't get your #1, #2, or #3 seat you could theoretically be seated in #4. If #1 open up (say a child who previous had it ranked the school at #2 and they got into their #1 later in the cycle) then you could ask for that seat again and be granted it because you are now the preferred student. Technically you lost #4 and gained #1. If you were to lose that #1 later in the algorithm then you could ask for #4 again and most likely get it back. This is called "deferred acceptance" and the trickiest part of the lottery to explain.

Under what circumstances would you not get the #4 seat back? If the seat was taken by another student with higher preference for that school?

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:
Show me where it is clearly stated that random lottery number trumps student's ranking of the school and, where all else is equal except the student's rank of the school, a higher random lottery number trumps student's ranking of school?

It's not clearly stated because there isn't anything at all about rank in the FAQ. Let me ask this back: If rank were so important -- more important than lottery number -- why did they leave it out of the FAQ?

I have no idea why they left it out. I only know what the admissions people at the 2 schools I'm most interested in told me and the person representing the lottery at the fair I went to said. They clearly, unequivically said "Rank matters and a student with everything else the same but with who ranked the school lower would lose to a student who ranked it higher".

For me, that's all I need to know, and I'm acting on that regardless of what anonymous people on the internet say. But I'm interested in the sources people are using to assert so strongly that random lottery number trumps rank.

Very strange. When I asked the Schools DC people at the ward 5 school fair, they did not tell me that. They stressed that it was important to put your true first choice in the #1 slot and that your chances of getting in to each school were independent of each other or rank.

Seems like every possibility being discussed here means you should put your true rank order, but if rank has nothing to do with your chances of getting in, why do they want you to rank it? Just to know what order to keep you/drop you as the slots fill up? The answer you got actually doesn't mention lottery number or rank, both of which makes your chances relevant to others (even though you can't control lottery number, which is assigned to you). But if the random lottery number has an impact, then what happens to you is still relevant to what happens to others.

Rank still matters.

If you don't love your IB/SIB/PROX school then don't rank it #1. Rank your favorite school #1 and increase your chances of getting into it. How do you increase your chances? Well, if you put your so-so IB/SIB/PROX school as #1 and get accepted then you won't even be considered for your dream school.

Isn't a 1 in 500 chance better than a 0 in 0 chance?