The party preference survey in May 2011 was conducted by telephone interviews of a national random probability sample consisting of 9 123 persons entitled to vote in the parliamentary election, without an upper age limit. Of these persons, 13.1 percent could not be reached (including unlisted telephone numbers and those without telephones), 3.0 percent were too ill to be interviewed and 16.5 percent did not want to be interviewed. The total non-response rate was 32.6 percent. A total of 6 147 interviews were conducted. In addition, some persons did not want to answer certain questions. The interviews were conducted by telephone during the period 1 - 26 May. The majority of the interviews were conducted during the first half of the measuring period. By 15 May, 79 percent of the interviews had been conducted.
The results are presented in intervals or point estimations ± margins of error. The interval that is created of the percent estimation ± margin of error here is a 95 percent confidence interval, which is an interval with a 95 percent probability including the actual value of the population, if no systematic errors occur.