Balinski–Young theorem: Difference between revisions

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{{cite book |title=Fair Representation: Meeting the Ideal of One Man, One Vote |last=Balinski |first=M |author2=Young HP |year=1982 |publisher=Yale Univ Pr |isbn=0-300-02724-9 |url-access=registration |url=https://archive.org/details/fairrepresentati00bali }}</ref><ref>
{{cite book |title=Fair Representation: Meeting the Ideal of One Man, One Vote |last=Balinski |first=M |author2=Young HP |year=1982 |publisher=Yale Univ Pr |isbn=0-300-02724-9 |url-access=registration |url=https://archive.org/details/fairrepresentati00bali }}</ref><ref>
{{cite book |title=Fair Representation: Meeting the Ideal of One Man, One Vote |edition=2nd |last=Balinski |first=M |author2=Young HP|year=2001 |publisher=Brookings Institution Press |isbn=0-8157-0111-X }}
{{cite book |title=Fair Representation: Meeting the Ideal of One Man, One Vote |edition=2nd |last=Balinski |first=M |author2=Young HP|year=2001 |publisher=Brookings Institution Press |isbn=0-8157-0111-X }}
</ref> The theorem shows that any possible method used to allocate the remaining fraction will necessarily fail to always follow quota. More precisely, their theorem states that there is no apportionment system that has the following properties <ref name=Stein2008>Stein JD. How Math Explains the World: A Guide to the Power of Numbers, from Car Repair to Modern Physics. Smithsonian. Apr 22, 2008. {{ISBN|9780061241765}}</ref> (as the example we take the division of seats between parties in a system of [[proportional representation]]):
</ref> The theorem shows that any possible method used to allocate the remaining fraction will necessarily fail to always follow quota. More precisely, their theorem states that there is no apportionment system that has the following 3 properties <ref name=Stein2008>Stein JD. How Math Explains the World: A Guide to the Power of Numbers, from Car Repair to Modern Physics. Smithsonian. Apr 22, 2008. {{ISBN|9780061241765}}</ref> (as the example we take the division of seats between parties in a system of [[proportional representation]]):
* It avoids violations of the [[W:quota rule | Quota Rule]]: Each of the parties gets one of the two numbers closest to its fair share of seats. For example, if a party's fair share is 7.34 seats, it must get either 7 or 8 seats to avoid a violation; any other number will violate the rule.
* It avoids violations of the [[W:quota rule | Quota Rule]]: Each of the parties gets one of the two numbers closest to its fair share of seats. For example, if a party's fair share is 7.34 seats, it must get either 7 or 8 seats to avoid a violation; any other number will violate the rule.
* It does not have the Alabama paradox: If the total number of seats is increased, no party's number of seats decreases.
* It has [[House monotonicity criterion | House monotonicity]]: If the total number of seats is increased, no party's number of seats decreases.
* It does not have the population paradox: If party A gets more votes and party B gets fewer votes, no seat will be transferred from A to B.
* It has [[W: Apportionment_paradox#Population_paradox | Population monotonicity]]: If party A gets more votes and party B gets fewer votes, no seat will be transferred from A to B.

== Specific Systems==


Methods may have a subset of these properties, but can't have all of them:
Methods may have a subset of these properties, but can't have all of them:
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* A method may be free of both the Alabama paradox and the population paradox. These methods are [[Highest averages method|divisor methods]],<ref name=Smith>{{cite web |url=http://rangevoting.org/Apportion.html |title=Apportionment and rounding schemes |last=Smith |first=WD}}</ref> and [[W:Huntington-Hill method|Huntington-Hill]], the method currently used to apportion House of Representatives seats, is one of them. However, these methods will necessarily fail to always follow quota in other circumstances.
* A method may be free of both the Alabama paradox and the population paradox. These methods are [[Highest averages method|divisor methods]],<ref name=Smith>{{cite web |url=http://rangevoting.org/Apportion.html |title=Apportionment and rounding schemes |last=Smith |first=WD}}</ref> and [[W:Huntington-Hill method|Huntington-Hill]], the method currently used to apportion House of Representatives seats, is one of them. However, these methods will necessarily fail to always follow quota in other circumstances.
* No method may always follow quota and be free of the population paradox.<ref name=Smith/>
* No method may always follow quota and be free of the population paradox.<ref name=Smith/>
* [[Largest remainder method | Largest remainder methods]] obey Quota Rules but have neither of the other two criteria. The [[Hamilton method]] of apportionment is actually a largest-remainder method which is specifically defined as using the Hare Quota

==History==


The division of seats in an election is a prominent cultural concern. In 1876, the United States [[W:1876 United States presidential election|presidential election]] turned on the method by which the remaining fraction was calculated. Rutherford Hayes received 185 electoral college votes, and Samuel Tilden received 184. Tilden won the popular vote. With a different rounding method the final electoral college tally would have reversed. However, many mathematically analogous situations arise in which quantities are to be divided into discrete equal chunks. The Balinski–Young theorem applies in these situations: it indicates that although very reasonable approximations can be made, there is no mathematically rigorous way in which to reconcile the small remaining fraction while complying with all the competing fairness elements.
The division of seats in an election is a prominent cultural concern. In 1876, the United States [[W:1876 United States presidential election|presidential election]] turned on the method by which the remaining fraction was calculated. Rutherford Hayes received 185 electoral college votes, and Samuel Tilden received 184. Tilden won the popular vote. With a different rounding method the final electoral college tally would have reversed. However, many mathematically analogous situations arise in which quantities are to be divided into discrete equal chunks. The Balinski–Young theorem applies in these situations: it indicates that although very reasonable approximations can be made, there is no mathematically rigorous way in which to reconcile the small remaining fraction while complying with all the competing fairness elements.

Revision as of 03:49, 27 January 2020

In 1983, two mathematicians, Michel Balinski and Peyton Young, proved that any method of apportionment will result in paradoxes whenever there are three or more parties (or states, regions, etc.).[1][2] The theorem shows that any possible method used to allocate the remaining fraction will necessarily fail to always follow quota. More precisely, their theorem states that there is no apportionment system that has the following 3 properties [3] (as the example we take the division of seats between parties in a system of proportional representation):

  • It avoids violations of the Quota Rule: Each of the parties gets one of the two numbers closest to its fair share of seats. For example, if a party's fair share is 7.34 seats, it must get either 7 or 8 seats to avoid a violation; any other number will violate the rule.
  • It has House monotonicity: If the total number of seats is increased, no party's number of seats decreases.
  • It has Population monotonicity: If party A gets more votes and party B gets fewer votes, no seat will be transferred from A to B.

Specific Systems

Methods may have a subset of these properties, but can't have all of them:

  • A method may follow quota and be free of the Alabama paradox. Balinski and Young constructed a method that does so, although it is not in common political use.[4]
  • A method may be free of both the Alabama paradox and the population paradox. These methods are divisor methods,[5] and Huntington-Hill, the method currently used to apportion House of Representatives seats, is one of them. However, these methods will necessarily fail to always follow quota in other circumstances.
  • No method may always follow quota and be free of the population paradox.[5]
  • Largest remainder methods obey Quota Rules but have neither of the other two criteria. The Hamilton method of apportionment is actually a largest-remainder method which is specifically defined as using the Hare Quota

History

The division of seats in an election is a prominent cultural concern. In 1876, the United States presidential election turned on the method by which the remaining fraction was calculated. Rutherford Hayes received 185 electoral college votes, and Samuel Tilden received 184. Tilden won the popular vote. With a different rounding method the final electoral college tally would have reversed. However, many mathematically analogous situations arise in which quantities are to be divided into discrete equal chunks. The Balinski–Young theorem applies in these situations: it indicates that although very reasonable approximations can be made, there is no mathematically rigorous way in which to reconcile the small remaining fraction while complying with all the competing fairness elements.

Related

Further Reading

References

  1. ↑ Balinski, M; Young HP (1982). Fair Representation: Meeting the Ideal of One Man, One Vote. Yale Univ Pr. ISBN 0-300-02724-9.
  2. ↑ Balinski, M; Young HP (2001). Fair Representation: Meeting the Ideal of One Man, One Vote (2nd ed.). Brookings Institution Press. ISBN 0-8157-0111-X.
  3. ↑ Stein JD. How Math Explains the World: A Guide to the Power of Numbers, from Car Repair to Modern Physics. Smithsonian. Apr 22, 2008. ISBN: 9780061241765
  4. ↑ Balinski, M; Young HP (1974). "A new method for congressional apportionment". Proceedings of the National Academy of Sciences. 71 (11): 4602–4606. doi:10.1073/pnas.71.11.4602. PMC 433936. PMID 16592200.
  5. ↑ a b Smith, WD. "Apportionment and rounding schemes".
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