Because Jefferson was home roughly 50 percent of the time since Sally's first and last pregnancy, the scenario is not unlike flipping a fair coin. However, flips of a fair coin are independent events. That is neither the case with Jefferson's stays at Monticello, which were not random (being at Monticello on a given day makes the likelihood of being there the next day greater), nor the case with Hemings's conception dates (getting pregnant one day makes it impossible to get pregnant the next).
