From Academic Kids
Parallel voting describes a mixed voting system where voters in effect participate in two separate elections using different systems, and where the results in one election have little or no impact on the results of the other. If one of the two election counts does have a substantial impact on the result of the other, then mixed member proportional voting may be a better description.
Under SM, a proportion of seats in the legislature are filled by FPP, with single member constituencies. The remainder are filled from party lists, with parties needing to have polled 5 per cent of the vote in order to achieve representation, as under the Additional Member System (AMS).
Unlike AMS, however, where party lists are used to achieve an overall proportional result in the legislature, under SM, proportionality is confined only to the list seats. Therefore, a party that secured say 5 per cent of the vote will have only 5 per cent of the list seats, and not 5 per cent of all the seats in the legislature.
The proportion of constituency seats compared to AMS seats is often but need not be 50:50.
Advantages and Disadvantages
SM allows smaller parties to secure representation in the legislature without having disproportionate power, as would be the case under an entirely proportional system. A criticism of proportional voting systems, is that the largest parties need to rely on the support of smaller ones in order to form a government. However, smaller parties are still disadvantaged as the larger parties still predominate.
Since FPP in single member constituencies are likely to lead to clear majorities, and thus "strong government", the extra seats that the big parties are likely to win as well are unnecessary for strong government. The opposition, which may only win seats in the SM part of the election may be too weak to ensure that the government is accountable, leading to less than good government.
Good governance requires both strong governments and reasonably strong oppositions, and the best system may be a mixture of Supplementary Members and Top-Up AMS Members, with a variable choice of proportions between the Single Member Constituency and the PR part of the election.
Countries where used
Taiwan (ROC) - constitutional amendment pending to begin this system in 2007