pyRCV2/docs/options.md

Options and advanced options

Preset

The preset dropdown allows you to choose from a hardcoded list of preloaded STV counting rules. These are:

• Scottish STV
• pyRCV STV-C: Our recommended rules for a computerised STV count
• Australian Senate STV
• Wright STV
• PRSA 1977

This functionality is not available on the Python command line.

Quota (-q/--quota, -c/--quota-criterion)

The quota dropdowns allow you to define the quota used in the election, and the quota criterion used to elect candidates. The quota may be set to:

• Droop: ⌊V/(S+1)⌋ + 1
• Droop (exact): V/(S+1) (also known as the Newland-Britton or Hagenbach-Bischoff quota)
• Hare: V/S
• Hare (exact): ⌊V/S⌋ + 1

where V is the number of votes, S is the number of seats and ⌊…⌋ denotes the floor function (round down to the nearest integer, discarding any remainder).

The quota criterion may be set to >= (candidates are elected if they meet or exceed the quota) or > (candidates are elected only if they strictly exceed the quota).

Note that the combination ‘>= Droop (exact)’ can result in more candidates meeting the quota than there are available vacancies, hence this particular combination is not recommended.

The default is ‘>= Droop’, which is the most common STV quota, but we recommend ‘> Droop (exact)’ as this prevents the quota being unnecessarily large when fractional surplus distribution methods (as in pyRCV2) are used.

Progressive quota (--prog-quota)

When this option is disabled (default), the quota is calculated once after all first-preference votes are allocated, and remains constant throughout the count. OpaVote calls this a ‘static threshold’.

When this option is enabled, the quota is recalculated at the end of every stage, and may decrease throughout the count as votes are exhausted. OpaVote calls this a ‘dynamic threshold’.

Bulk election (--no-bulk-election)

When bulk election is enabled (default), all remaining candidates are declared elected in a single round once the number of not-excluded candidates exactly equals the number of vacancies to fill. Further surplus distributions are not performed. This is typical of most STV rules.

When bulk election is disabled, surpluses continue to be distributed even once the number of not-excluded candidates exactly equals the number of vacancies to fill. Once there are no more surpluses to distribute, the candidates are declared elected, one by one, in descending order of votes. This ensures that only one candidate is ever elected in each round and the order of election is well-defined, which is required e.g. for some affirmative action rules.

Numbers (-n/--numbers), Decimal places (--decimals)

This dropdown allows you to select how numbers (vote totals, etc.) are represented internally in memory. The options are:

• Fixed: Numbers are represented as fixed-precision decimals, up to a certain number of decimal places (default: 5).
• Rational: Numbers are represented exactly as fractions, resulting in the elimination of rounding error, but increasing computational complexity when the number of surplus transfers is very large.
• Native float: Numbers are represented as floating-point numbers. This is fast, but not recommended as unexpectedly large rounding errors may be introduced in some circumstances.
• Native int: Numbers are represented as integers. Fractions are discarded.

Note that, during some calculations, in order to reduce rounding errors, ballot weights and intermediate may be represented internally as fractions irrespective of this option.

Surplus order (-s/--surplus-order)

This dropdown allows you to select in what order surpluses are distributed:

• By size (default): When multiple candidates exceed the quota, the largest surplus is transferred (even if it arose at a later stage of the count).
• By order: When multiple candidates exceed the quota, the surplus of the candidate elected first is transferred (even if it is smaller than another). Candidates are always declared elected in descending order of number of votes.

Some STV counting rules provide, for example, that ‘no surplus shall be transferred before a surplus that arose earlier in the counting whether larger or not’ (PRSA 1977). In this case, the option ‘By order’ should be selected.

Method (-m/--method)

This dropdown allows you to select how ballots are transferred during surplus transfers or exclusions. The 2 recommended methods are:

• Weighted inclusive Gregory: During surplus transfers, all applicable ballot papers of the transferring candidate are examined. Transfers are weighted according to the weights of the ballot papers.
• Wright STV: Same as weighted inclusive Gregory, but when a candidate is excluded, the count is reset from the beginning (minus the excluded candidate).

Other methods are supported, but not recommended:

• Unweighted inclusive Gregory: During surplus transfers, all applicable ballot papers of the transferring candidate are examined. Transfers are not weighted, and each ballot paper has equal value in the calculation.
• Exclusive Gregory (last bundle): During surplus transfers, only the ballot papers received in the last transfer are examined. Transfers are not weighted.

Other surplus transfer methods, such as non-fractional transfers (e.g. random sample) are not supported at this time.

Papers to examine in surplus transfer (--transferable-only)

• Include non-transferable papers (default): When this option is selected, all ballot papers of the transferring candidate are examined. Non-transferable papers are always exhausted at the relevant transfer values.
• Use transferable papers only (CLI: --transferable-only): When this option is selected, only transferable papers of the transferring candidate are examined. Non-transferable papers are exhausted only if the value of the transferable papers is less than the surplus.

Exclusion method (--exclusion)

• Exclude in one round (default): When excluding a candidate, transfer all their ballot papers in one round.
• Exclude by parcel (by order): When excluding a candidate, transfer their ballot papers one parcel at a time, in their order each was received. Each parcel forms a separate round, i.e. if a transfer allows another candidate to meet the quota criterion, no further papers are transferred to that candidate.
• Exclude by value: When excluding a candidate, transfer their ballot papers in descending order of accumulated transfer value. Each transfer of all ballots of a certain transfer value forms a separate round.

Ties (-t/--ties)

This dropdown allows you to select how ties (in surplus transfer or exclusion) are broken. The options are:

• Backwards: Ties are broken according to which tied candidate had the most/fewest votes at the end of the last stage where one tied candidate had more/fewer votes than the others, if such a stage exists.
• Random: Ties are broken at random (see Random seed).
• Prompt: The user is prompted to break the tie.

Multiple tie breaking methods can be specified. If the first method cannot resolve the tie, the next is tried, and so on. In the web application, 2 options are available (‘Backwards then random’ and ‘Random’). On the Python command line, the --ties option can be specified multiple times (e.g. --ties backwards --ties random).

Random seed (--random-seed)

This option allows you to input an arbitrary value to seed the deterministic random number generator used to break ties when Ties is set to Random. When the same seed value is used, ties will always be broken in the same way, allowing the breaking of ties to be independently verified.

The default value is the current date, formatted YYYYMMDD.

The algorithm used by the random number generator is specified at rng.md.