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Why polls in Brazil show different results

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A Quaest poll published on Wednesday showed a gap of just 8 points between the presidential frontrunners, former President Luiz Inácio Lula da Silva and incumbent Jair Bolsonaro — a much tighter margin than the 15 points in an Ipec poll published on Monday and the 11 points flagged by Datafolha last Friday. The polls were conducted within days of each other and with similar sample sizes. Ipec surveyed 2,515 people, while Quaest interviewed 2,000 voters.

So how come results are so different?

One of the reasons is the institutes’ sampling. For instance, in the Quaest poll, 21 percent of the voters are aged 60 and up — compared with 17 percent for Ipec. Polls have shown that younger voters lean much more heavily towards Lula than older ones.

Income is also a significant factor. Well over half (57 percent) of voters polled by Ipec said their household makes up to twice Brazil’s minimum wage, compared with just 38 percent in the Quaest poll. Poorer voters tend to lean towards Lula and the Workers’ Party.

In the Ipec poll, only 13 percent of respondents’ household income tops five times the minimum wage, compared with 22 percent in the Quaest sample.

Antonio Lavareda, president of pollster Ipespe, calculated what Ipec’s result would be if it used the same income proportions as Quaest did. “BINGO! The Ipec result would be in the first round: Lula, 43 percent, and [Mr.] Bolsonaro, 35 percent. That is, the same difference of 8 points,” he tweeted.

Outdated census data explains the differences in the weighting given to different categories in pollsters’ samples. Field agents only just started collecting data for the delayed 2020 Census in August of this year.

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