PREVAIL creates perfect proportionality by transferring some seats to parties that did not come first in the constituency. Each party that has the highest vote in at least one constituency (called an elected party) is given a quota of seats that matches its share of the vote amongst elected parties. A party that gained more seats under FPTP than its quota is said to be over quota. Likewise a party can be on quota or under quota. PREVAIL will transfer seats from over quota parties to under quota parties until all parties  are on quota.

PREVAIL is concerned with the degree of proportionality amongst parties. An independent candidate gaining the highest vote in a constituency is immediately elected just as under First past the Post. This is the only way an independent candidate can win a seat. Independent candidates are not involved in transfers

The quota system ensures that every vote cast for a party that is elected is used, whether or not the party came first in a particular constituency. This is in marked contrast to First Past the Post where votes cast only count if the party wins in a seat.

Calculating the quotas

A seat is called a party seat if the highest vote was obtained by a party candidate rather than an independent candidate. The total number of party seats is N. The popular votes (PV) for each elected party are added to give the elected party total vote (TV). Note that TV is less than the total number of valid votes cast in the election because votes for independent candidates are ignored.

Each party is given a quota of seats in proportion to its popular vote:

N*PV/TV

For each party, this number is rounded to the nearest whole number. This rounding may cause the total quota to differ slightly from N. The quota of the party with the largest popular vote is adjusted to correct this. In this way the proportionality is least affected.

PREVAIL will then identify which parties are over, on or under quota.

A simple measure of the degree of disproportionality is the number of seats, M, requiring to be transferred to achieve perfect proportionality. This can be expressed as a fraction of the total number number of party seats, N. Expressed as a percentage this is:

100*M/N

A percentage of zero constitutes perfect proportionality.

Seat allocation

The party that obtained the highest number of votes in a constituency is called the First Party; the party that gains the second highest number of votes is called the Second Party; the party that gains the third highest number of votes is called the Third Party; likewise for fourth and subsequent parties.

Initially PREVAIL allocates each seat as in FPTP, i.e. to the first party or to an independent candidate who came first. Seats are then transferred one by one from parties that are over quota to parties that are under quota. Seats are not transferred from parties that are on quota. If a party was first in only one seat then that seat cannot be transferred.

The transfer process

The seats in which an over quota party came first are arranged in decreasing order of percentage vote. The party retains as its quota its best seats down to the quota cut. All seats below the quota cut are transferred. 

For over quota parties, the seats below the quota cut are combined into a single list. This is the transfer list.  

Seats are transferred to one under quota party at a time until that party reaches quota. Transfers begin with the party that requires the least number of seats to bring it up to quota and ends with the party needing the greatest number of seats. If two or more parties need the same number of transfers they are ordered according to popular vote with the party having the lowest going first. 

An under quota party initially receives seats in which it came second. If this is not sufficient to bring it up to quota then it receives seats in which it came third and so on. 

To transfer seats to a party that came second, the transfer list is ranked in decreasing order of that party's second place votes. Seats are transferred beginning at the top and transfer continues until either the party achieves its quota or the bottom of the list is reached. If necessary transfers are now made to seats in which the party came third. Now the transfer list is ranked in decreasing order of that party's third place votes. This procedure continues with fourth place votes and so on until the party has reached its quota. Transfers then move to the next under quota party as described above.

As transferring proceeds seats some will have already been transferred and are no longer available as the list is worked down.

It is possible, particularly near the end of the transfer process, that an under quota party may not have stood in a constituency where it needs a transfer. In this case a swap can be made with another under quota party that did stand in the seat.  The order in which parties participate in the transfer process, from smallest shortfall to largest, helps to eliminate this possibility. 

Although the vote obtained by the party allocated a seat is not a particular priority for PREVAIL, the transfer process seeks to maintain as high as possible percentage vote of the seat receiving the transfer. In many cases this percentage vote may be only a point or so below that of the party which came first though in other cases it can be very small.   

Further points

Under PREVAIL the vast majority of votes have impact and are not wasted as in FPTP. Only votes cast for a party that does not come first in any seat (other than those for a party elected under PREVAIL Plus) can be said to be wasted. This is typically a very small percentage.

A party seat allocated under PREVAIL belongs to the party which then retains it for the duration of Parliament. There are no by-elections.  Should the member need to be replaced, for whatever reason, the party will nominate a new member. This is a common practice under PR systems. A similar scheme operates should an independent member need to be replaced: a successor, who will continue to function as an independent, will be named prior to the election.

Notwithstanding the philosophy of PREVAIL, a concern amongst voters may be that their seat may be transferred to a party with widely different views to their own. The constituency can exert some influence over this. It can elect a candidate who commands a high percentage of the vote (to be above the quota cut) or an independent candidate. 

Tactical voting has increased in popularity in recent years. A voter who normally votes for party B votes instead for party C in order to prevent party A from winning. The objective is either to reduce the power of party A in Parliament or to prevent a particular local candidate or party from gaining the seat, as discussed above.  Under FPTP, if tactical voting succeeds it will reduce the number of seats held by party A. This cannot happen under PREVAIL because the quota for party A depends entirely on its own votes and cannot be influenced by voters who were not going to vote for party A anyway. Because of the transfer process, the outcome of tactical voting is very difficult to predict. The advice to voters would be to concentrate on Parliament when they vote. 

Constituency Management