MADRID (Reuters) - Four opinion polls published since Spain’s prime minister called a general election for April 28 have predicted widely different outcomes, illustrating how the emergence of new parties, and in particular the far-right Vox, has upset political forecasting.

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In December, pollsters failed to predict the success of the anti-immigration Vox, which won 12 seats in a regional election in Andalusia in the first electoral win for a far-right party in Spain in over four decades.

The four surveys published since last Friday’s election announcement by Prime Minister Pedro Sanchez agree on one thing: Vox will get lawmakers elected to the national parliament. But their predictions range from 16 to 46 seats.

Similarly, all polls see Sanchez’s Socialists leading the election and getting more seats than in the previous election in 2016, while falling well short of a majority in the 350-seat parliament.

But the GAD3, Gesop, Metroscopia and NCReport polls disagree on who would have a shot at a coalition deal - the conservative People’s Party (PP) with center-right Ciudadanos and Vox, or the Socialists with Ciudadanos.

To complicate matters, government sources do not rule out a Socialist deal with Ciudadanos, but the center-right party officially says it does not want one at a national level, although it does not exclude such deals for municipal elections.

“We have a context of high volatility,” said Lluis Orriols, a political science professor at Carlos III university, saying that was making it harder for pollsters to interpret results.

The electoral system gives the party with the most votes a bonus in terms of seats. There is also a gray area around the 10-15 percent of votes where there can be a huge difference in the number of seats allocated, while parties getting less than 10 percent of votes see their number of lawmakers drop sharply.

For instance, in 2015 Ciudadanos obtained 13.9 percent of the vote and 40 seats. In an election the following year, it obtained 13.1 percent of the vote but eight fewer seats.

Ciudadanos, Vox and the far-left Podemos are all now potentially in that 10-15 percent gray area.

“A word of advice: more caution than ever with the polls,” Narciso Michavila, the head of GAD3 pollsters, told ABC newspaper.

He also noted that the last polls before the election will be published during Easter holidays, at a time when many voters will be focused on issues other than the elections.