Balinski–Young theorem: Difference between revisions
Moved latter part of History to Specific systems, as it didn't relate to history, and added note about fractional voting power.
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(Moved latter part of History to Specific systems, as it didn't relate to history, and added note about fractional voting power.) |
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{{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 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 [[
* It has [[House monotonicity criterion | House monotonicity]], avoiding the [[Alabama paradox]]: 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.
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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 the quota rule in some elections.
* No method may always follow quota and be free of the population paradox.<ref name=Smith/>
* [[Largest remainder method | Largest remainder methods]] obey
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.
==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.
== Notes ==
The quota rule is incompatible with immunity to [[vote management]]. See [[Divisor method#Notes]] for an example.
For [[proportional representation]], the paradox can theoretically be sidestepped by giving party representatives slightly different voting power to correct for any error. For instance, if 13.57% of the voters voted for party A, the total voting power of party A's representatives could be set to 13.57% of the total. However, it makes counting votes much more opaque. This solution is not employed by any current political assembly.
==Related==
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