Under SM, a proportion of seats in the legislature are filled by pluralities in single member constituencies. The remainder are filled from party lists, with parties often needing to have polled a certain amount, typically a small percentage, in order to achieve representation, as is common in many proportional systems.
Unlike Mixed Member Proportional, 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% of the vote will have only 5% of the list seats, and not 5% of all the seats in the legislature.
The proportion of list seats compared to total seats ranges widely, from 37.5% in Japan to 18.7% in South Korea to 68.7% in Armenia.
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. In countries where there is one dominant party and a divided opposition, the proportional seats may be essential for allowing an effective opposition.
Since plurality voting in single member constituencies is 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.
Countries like Japan, Russia and Thailand adopted a parallel system as a means by which incentives for greater party cohesiveness could be injected. The party is sure to elect the candidates at the top of its list, guaranteeing safe seats for the leadership. By contrast, under the MMP system a party that does well in the local seats will not need or receive any compensatory list seats, so the leadership has to run in the local seats.