In the 2016 US presidential election, Hillary Clinton looked set to secure a landslide victory over Donald Trump. But Trump surged ahead, leaving analysts baffled.
Compared with the UK and the US, where election results from different constituencies are combined in different ways, Swedish polls are more reliable, says Annika Fredén, associate professor at Lund University.
But that doesn't mean they are perfect or that certain voter groups haven't been over- or underrepresented.
If you go back in time, the Green Party always got higher numbers in opinion polls than it did in the election, says Dan Hedlin, professor of statistics at Stockholm University.
Opposite effect
The explanation lies in the Green Party's previous ability to be easily liked by many, he believes.
The institute doesn't just ask the question: which party would you vote for if there were an election today? For those who don't know, the next question is: which party is closest to hand? If you say the Green Party, you are considered an environmentalist.
The opposite effect has characterized the Sweden Democrats.
Their sympathizers are less willing to participate in surveys than others. This means that they are underestimated. Now institutes have methods to counteract this, but it is not always enough.
Normally, larger opinion polls in Sweden have a margin of error of around two percentage points, according to Hedlin.
Psychological effect
However, individual opinion polls may show a larger margin of error.
An opinion poll from last spring showed that the Liberals passed the four percent threshold after the announcement that they would sit in government together with the Sweden Democrats. Later, Demoskop's methods were criticized for using a self-recruiting panel.
No one wants their vote to be wasted. Therefore, occasional polls above four percent can have a psychological effect and influence voters' propensity to vote in support.
Especially if they come to a critical situation when many people are weighing in.
But it depends on which party it is about.
If it is a party that has an established position, and is part of a coalition, I have been able to see that even low opinion figures such as 2.5 percent cause voters to vote in support, she says, mentioning the Christian Democrats as an example.
