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The Huntington–Hill method of apportionment assigns seats by finding a modified divisor D such that each constituency's priority quotient (its population divided by D), using the geometric mean of the lower and upper quota for the divisor, yields the correct number of seats that minimizes the percentage differences in the size of subconstituencies.^{[1]} When envisioned as a proportional electoral system, it is effectively a highest averages method of partylist proportional representation in which the divisors are given by , n being the number of seats a state or party is currently allocated in the apportionment process (the lower quota) and n + 1 is the number of seats the state or party would have if it is assigned to the party list (the upper quota). Although no legislature uses this method of apportionment to assign seats to parties after an election, it was considered for House of Lords elections under the illfated House of Lords Reform Bill.^{[2]}
The method is how the United States House of Representatives assigns the number of representative seats to each state – the purpose for which it was devised – and the Census Bureau calls it the method of equal proportions.^{[3]} It is credited to Edward Vermilye Huntington and Joseph Adna Hill.^{[4]}
Allocation
In a legislative election under the Huntington–Hill method, after the votes have been tallied, the qualification value would be calculated. This step is necessary because in an election, unlike in a legislative apportionment, not all parties are always guaranteed at least one seat. If the legislature concerned has no exclusion threshold, the qualification value could be a predefined quota, such as the Hare, Droop, or Imperiali quota.
In legislatures which use an exclusion threshold, the qualification value would be equipotent to the threshold, that is:
where
 total votes is the total valid poll; that is, the number of valid (unspoilt) votes cast in an election.
 total seats is the total number of seats to be filled in the election.
Every party polling votes equal to or greater than the qualification value would be given an initial number of seats, again varying if whether or not there is a threshold:
In legislatures which do not use an exclusion threshold, the initial number would be 1, but in legislatures which do, the initial number of seats would be:
with all fractional remainders being rounded up.
In legislatures elected under a mixedmember proportional system, the initial number of seats would be further modified by adding the number of singlemember district seats won by the party before any allocation.
Determining the qualification value is not necessary when distributing seats in a legislature among states pursuant to census results, where all states are guaranteed a fixed number of seats, either one (as in the US) or a greater number, which may be uniform (as in Brazil) or vary between states (as in Canada).
It can also be skipped if the HuntingtonHill system is used in the nationwide stage of a national remnant system, because the only qualified parties are those which obtained seats at the subnational stage.
After all qualified parties or states received their initial seats, successive quotients are calculated, as in other Highest Averages methods, for each qualified party or state, and seats would be repeatedly allocated to the party or state having the highest quotient until there are no more seats to allocate. The formula of quotients calculated under the HuntingtonHill method is
where:
 V is the population of the state or the total number of votes that party received, and
 s is the number of seats that the state or party has been allocated so far.
Example
Even though the Huntington–Hill system was designed to distribute seats in a legislature among states pursuant to census results, it can also be used, when putting parties in the place of states and votes in place of population, for the mathematically equivalent task of distributing seats among parties pursuant to an election results in a partylist proportional representation system. A partylist PR system requires large multimember districts to function effectively.
In this example, 230,000 voters decide the disposition of 8 seats among 4 parties. Unlike the D'Hondt and SainteLaguë systems, which allow the allocation of seats by calculating successive quotients right away, the Huntington–Hill system requires each party or state have at least one seat to avoid a division by zero error. In the U.S. House of Representatives, this is ensured by guaranteeing each state at least one seat; in a singlestage PR election under the Huntington–Hill system, however, the first stage would be to calculate which parties are eligible for an initial seat.
This could be done by excluding any parties which polled less than a predefined quota, and giving every party which polled at least the quota one seat.
Denominator  Votes  Is the party eligible or disqualified?  

Hare  Droop  Imperiali  
Party A  100,000  Eligible  Eligible  Eligible 
Party B  80,000  Eligible  Eligible  Eligible 
Party C  30,000  Eligible  Eligible  Eligible 
Threshold  28,750  25,556  23,000  
Party D  20,000  Disqualified  Disqualified  Disqualified 
In this case, the qualified parties stay the same regardless of quota.
Each eligible party is assigned one seat. With all the initial seats assigned, the remaining five seats are distributed by a priority number calculated as follows. Each eligible party's (Parties A, B, and C) total votes is divided by 1.41 (the square root of the product of 1, the number of seats currently assigned, and 2, the number of seats that would next be assigned), then by 2.45, 3.46, 4.47, 5.48, 6.48, 7.48, and 8.49. The 5 highest entries, marked with asterisks, range from 70,711 down to 28,868. For each, the corresponding party gets another seat.
For comparison, the "Proportionate seats" column shows the exact fractional numbers of seats due, calculated in proportion to the number of votes received (For example, 100,000/230,000 × 8 = 3.48). If the "Total Seats" column is less than the "Proportionate seats" column (Parties C^{[a]} and D in this example) the party is underrepresented. Conversely, if the "Total Seats" column is greater than the "Proportionate seats" column (Parties A and B in this example) the party is overrepresented.^{[b]}
Denominator  1.41  2.45  3.46  4.47  5.48  6.48  7.48  8.49  Initial seats 
Seats won (*) 
Total Seats 
Proportionate seats^{[c]} 

Party A  70,711*  40,825*  28,868*  22,361  18,257  15,430  13,363  11,785  1  3  4  3.5 
Party B  56,569*  32,660*  23,094  17,889  14,606  12,344  10,690  9,428  1  2  3  2.8 
Party C  21,213  12,247  8,660  6,708  5,477  4,629  4,009  3,536  1  0  1  1.0 
Party D  Disqualified  0  0.7 
If the number of seats was equal in size to the number of votes cast, this method would guarantee that the apportionments would equal the vote shares of each party.
In this example, the results of the apportionment is identical to one under the D'Hondt system. However, as the District magnitude increases, differences emerge: all 120 members of the Knesset, Israel's unicameral legislature, are elected under the D'Hondt method^{[d]}. Had the Huntington–Hill method, rather than the D'Hondt method, been used to apportion seats following the elections to the 20th Knesset, held in 2015, the 120 seats in the 20th Knesset would have been apportioned as follows:
Party  Votes  Huntington–Hill  D'Hondt^{[d]}  +/–  

(hypothetical)  (actual)  
Last Priority^{[e]}  Next Priority^{[f]}  Seats  Seats  
Likud  985,408  33408  32313  30  30  0  
Zionist Union  786,313  33468  32101  24  24  0  
Joint List  446,583  35755  33103  13  13  0  
Yesh Atid  371,602  35431  32344  11  11  0  
Kulanu  315,360  37166  33242  9  10  –1  
The Jewish Home  283,910  33459  29927  9  8  +1  
Shas  241,613  37282  32287  7  7  0  
Yisrael Beiteinu  214,906  39236  33161  6  6  0  
United Torah Judaism  210,143  38367  32426  6  6  0  
Meretz  165,529  37013  30221  5  5  0  
Source: CEC 
Compared with the actual apportionment, Kulanu would have lost one seat, while The Jewish Home would have gained one seat.
Footnotes
 ^ Party C's proportion is actually 1.04
 ^ While this example favors the largest parties (Parties A and B), if a different number of seats were apportioned, other parties would be favored. In short, the largest party is not always favored.
For example, if there were 12 seats instead of 8, then Party C would be the only overrepresented party (since Party D would have qualified) with two full seats while proportionately deserving only 1.6 seats.  ^ This proportionality is based on the total votes. If instead it was based on the qualified votes (i.e., reducing the total 230,000 votes by the disqualified 20,000 votes for Party D), the proportionate seats would be: Party A  3.8 seats, Party B  3.0 seats, and Party C  1.1 seats.
 ^ ^{a} ^{b} The method used for the 20th Knesset was actually a modified D'Hondt, called the BaderOfer method. This modification allows for spare vote agreements between parties.^{[5]}
 ^ This is each party's last priority number which resulted in a seat being gained by the party. Likud gained the last seat (the 120th seat allocated). Each priority number in this column is greater than any priority number in the Next Priority column.
 ^ This is each party's next priority number which would result in a seat being gained by the party. Kulanu would have gained the next seat (if there were 121 seats in the Knesset). Each priority number in this column is less than any priority number in the Last Priority column.
References
 ^ "Congressional Apportionment". NationalAtlas.gov. Archived from the original on 20090228. Retrieved 20090214.
 ^ Draft House of Lords Reform Bill: report session 201012, Vol. 2. Google Books. 23 April 2012. ISBN 9780108475801. Retrieved 6 November 2017.
 ^ "Computing Apportionment". United States Census Bureau. Retrieved 20210426.
 ^ "The History of Apportionment in America". American Mathematical Society. Retrieved 20090215.
 ^ "With BaderOfer method, not every ballot counts". The Jerusalem Post. Retrieved 20210504.