Post-Mortem after the 2020 Presidential Election
The discussion below was on the website before the 2020 Presidential election. The last data collected was on November 2, 2020. These data are used for the test of the ranking assumption.The Democrats won Georgia, and so according to the ranking below and the ranking assumptionn they should have won all the states ranked above Georgia, reg. They in fact lost North Carolina and Florida. Georgia and Maine 02 were tied, and it is not clear how this should be treated. If this is counted as half an error, then the total number of errors is 2.5 for the ranking assumptoin. Predictit made an error regarding Georgia.
Background
Near the end of the 2020 election I collected market probabilities by state from Predictit on various days, the last day being the day before the election, November 2, 2020. The November 2 data were used to test the ranking assumption. The test is a test of the joint hypothesis that the Predictit market probabilities are right and the ranking assumption is right. The data as they were collected are presented in Table 1. The calculations that are behind the numbers in Table 1 are in earlier Predictit data for the 2020 Presidential election.Data collected from Predictit at 7pm EST on November 2, 2020
On this date, 18 states had market probabilities on the Predictit website less than 90 percent and greater than 10 percent for the Democrats winning the state. The 18 states, ranked by market probabilities for the Democratic candidate, are:
| state | prob | votes | sumvotes | |
| New Mexico | 88 | 5 | 209 | |
| Maine not 02 | 88 | 3 | 212 | |
| New Hampshire | 74 | 4 | 216 | |
| Minnesota | 73 | 10 | 226 | |
| Nevada | 71 | 6 | 232 | |
| Wisconsin | 69 | 10 | 242 | |
| Michigan | 69 | 16 | 258 | |
| Nebraska 02 | 68 | 1 | 259 | |
| Pennsylvania | 60 | 20 | 279 | pivot |
| Arizona | 52 | 11 | ||
| North Carolina | 47 | 15 | ||
| Florida | 43 | 29 | ||
| Maine 02 | 41 | 1 | ||
| Georgia | 41 | 16 | ||
| Ohio | 30 | 18 | ||
| Texas | 29 | 38 | ||
| Iowa | 25 | 6 | ||
| Alaska | 11 | 3 | ||
64 = Predictit market-based probability that the Democrats win the presidential election.
"sumvotes" is the sum of the electoral votes of all the states ranked above the state plus the state's vote. 270 votes are needed to win. You can see that Pennsylvania is the pivot state. If Biden takes Pennsylvania and all the states ranked above it, he gets 279 votes. Of the states ranked below Pennsylvania, he could also win by not taking Pennsylvania, but taking Arizona, North Carolina, or Florida. This would, of course, violate the ranking assumption.
According to the ranking assumption, the probability that the Democrats get a majority in the electoral college is the probability that they win the pivot state, Pennsylvania, which is 60. On the Predictit website, the market-based probability that the Democrats win the presidential election (in the electoral college) is 64, close to 60. So the market participants are roughly using the ranking assumption.