Why opinion polls can’t be trusted in Argentina

The Kirchner family was near the peak of its power back in 2009, and a majority of opinion polls in Argentina expected them to win that year’s congressional race in the midterms.

But they were proved wrong that Sunday night. Néstor Kirchner had suffered his first electoral loss since winning Argentina’s presidency in 2003, as the fallout of a massive conflict with farmers and the 2008 subprime crash proved too much to take at once.

On a closer look, the fact that the mistaken pollsters had been hired by government allies pointed to a clear conflict of interest. So, for the 2011 presidential vote, when polls said Cristina Kirchner would win in a landslide, without the need for a runoff, most people ignored them.

But Mrs. Kirchner did sail to victory, with 50 percent of the vote in the 2011 presidential primaries. The second-placed coalition barely reached 12 percent, all but confirming she would be the next president.

Interestingly, one of the few pollsters that had got 2009 right, Poliarquía, hired by the center-right newspaper La Nación — a strong critic of the Kirchners and historically in line with Argentina’s elites — did not publish its polls in the buildup to the 2011 elections, only showing them in private to a few select clients.

The combined story of both elections showed that pollsters on both sides of Argentina’s political divide could manipulate public opinion by altering or silencing their results.

The errors continued in 2015, when pollsters failed to predict the right-wing Mauricio Macri’s surge to end only 3 points behind Peronist...