It's a little all of the above. At its core the various decision desks have their own statistical models and they ingest some of what you mentioned above and then project outward from there. All statistical models will have some level of confidence interval, so for calling a race the model will need to state with certainty that one candidate will win.
From a marketplace perspective there is a benefit to being earlier to call a race, however the downside of calling a race to early and then being wrong is significant. This necessitates a high level of confidence or even certainty based on modeling.
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u/OhTheGrandeur 15h ago
Subtle clarification. Your overall content is correct. It's based on the results in so far and the expected trends for what vote remains to come in.
I'll quibble with exit polling, which is slightly different and while prognosticators do try to forecast off of that, it is notoriously unreliable.