The t-distribution in FiveThirtyEight's model mostly makes a difference for very low-probability events.

Judged as of 2016. The claim is that using a fat-tailed t-distribution rather than a normal distribution in FiveThirtyEight's election model changed results mainly for very low-probability outcomes, i.e. in the tails. True: FiveThirtyEight's methodology documents, together with standard statistical properties, show the t-distribution's effect is concentrated in tail probabilities (e.g. a candidate with a small chance of winning a state) and is small for outcomes near the middle. Mostly true: the effect is concentrated in the tails but also has some non-trivial effect on moderate probabilities. Mixed: the effect is spread fairly evenly between tail and moderate probabilities. Mostly false: the t-distribution materially changed moderate probabilities as well. False: the model did not use a t-distribution, or the effect was mainly on moderate probabilities.

Set before the research began.

1. E1 What’s Wrong With 538? HuffPost · 7 November 2016

   “I am not questioning any specific assumption that 538 makes, from their state-by-state correlations to their use of a t-distribution to create “fat tails” in their probability distributions (i.e., higher likelihood of otherwise obscure events).”

2. E2 What’s Wrong With 538? HuffPost · 7 November 2016

   “Perhaps Nate Silver can say that he’s gotten all of these improbable outcomes because of his use of the t-distribution rather than a normal distribution to accentuate the tails of the model.”

3. E3 Accurate Prediction of Electoral Outcomes arXiv (academic preprint by outside authors)

   “If we study the histogram in Figure 24, it is not clear that fat-tailed distributions are relevant here, unless we believe there are non-trivial probabilities of a landslide.”

4. E4 Accurate Prediction of Electoral Outcomes arXiv (academic preprint by outside authors)

   “The probabilities provided by the Student T regression as well as the Hierarchical Model were very similar to the CAPM Model.”

An independent 2016 HuffPost piece confirms that FiveThirtyEight used a t-distribution to create "fat tails" in its probability distributions, which it describes as a "higher likelihood of otherwise obscure events" [E1]. The same piece describes the t-distribution as a way to "accentuate the tails of the model" [E2]. That matches the claim that the effect falls mainly on low-probability outcomes. No FiveThirtyEight methodology document is in the evidence, and nothing measures how much the choice changed moderate probabilities, so the claim cannot be fully confirmed. The arXiv preprint [E3, E4] studies a different model, so it is not used.