Picking portfolios: How it works

BBC News Online looks at the mathematical voting formula which will be used to allocate ministerial portfolios at some point after the assembly election.

Northern Ireland's voters used the Single Transferable Vote system (STV) to elect members to the new assembly.

If a power-sharing executive is formed, it will have four DUP ministers, three Sinn Fein, two UUP and one SDLP.

Ministerial seats will be shared out by using the d'Hondt system.

Used widely across Europe, it is named after a 19th century Belgian lawyer and is based on the "highest average method".

Employing d'Hondt in Northern Ireland was a stipulation of the Good Friday Agreement.

It was felt to be a suitable election mechanism for use in a divided society, aimed at ensuring cross-community representation.

However, the system tends to favour larger parties.

While, it is not normally used to form governments or administrations, the mechanism is used by Switzerland and Belgium to elect members to its federal bodies.

The mathematical formula seeks to reflect the strength of a party's total support by taking into account its share of votes in relation to the number of seats already won.

It means the average number of votes required to win one seat is almost the same for each party.

It uses a method of counting votes to ensure that each party receives a level of seats or ministerial posts, related to its level of popular support - but not at the expense of other parties which could lose out through other systems such as "first past the post" or proportional representation.

The mechanism was thought ideal for the Northern Ireland Assembly because it ensures that the body works as a power-sharing institution.

As the largest party, the DUP has the first choice of ministry, followed by Sinn Fein, the second largest party, and so on.

The system differs from the single transferable vote (STV) in that it does not use a quota or formula to allocate seats or posts.

Rather, these are allocated singularly and one after another.

A party's total vote is divided by a certain figure, which increases as it wins more seats.

As that dividing figure increases, the party's total in succeeding rounds gets smaller. This allows parties with lower initial totals to win seats.

In the first round of vote-counting, the dividing figure is one and therefore has no effect.

However, the figure in subsequent rounds is the total number of seats gained plus one.

Also under d'Hondt, the parties nominate committee chairs and committee members of the assembly.

The parties can exclude themselves from the executive committee, and if it withdraws support from the committee its seats can be re-distributed under d'Hondt.

In the election for the Lord Mayor of London, the d'Hondt mechanism was used to calculate additional seats.

It is also used in the European Parliament to distribute committee chairs and vice-chairs among parties.