In "The Lottery," what normal law of probability has been suspended in the story? Granting this initial implausibility, does the story proceed naturally?
To me, the normal law of probability broken in this story is that a small town in the United States in the late 1940s would be so superstitious and out of sync with ideas of science and modernity that it would imagine a human sacrifice would ensure a good corn crop yield. This is the kind of custom that one might expect as a holdover from pagan or even medieval times in a remote corner of Europe. But given that the first English settlements were established in the United States in the early seventeenth century, it seems unlikely that such a custom, at that late date, would have come over from Europe. It seems even more unlikely that such a custom would develop independently in the US. And it seems to defy credibility that it would persist so far into the twentieth century without a majority of the villagers protesting it—or deciding to leave town for a safe factory job in Detroit or Boston. After all, we are told the village only numbers 300, so the risk that oneself or a close family member would get chosen is relatively high.
Nevertheless, suspension of disbelief is a part of reading literature. It is a testament to Jackson's strength as a writer that in her hands this custom of stoning a victim chosen by lottery seem plausible in twentieth-century America.
In "The Lottery," it is highly improbable that Old Man Warner would survive into old age. To illustrate this point, consider that the lottery takes place every year so Old Man Warner has survived seventy-seven lotteries. This is an impressive feat, made all the more improbable by the village's relatively small population. With only three hundred residents, Old Man Warner has continually overcome the high odds of being chosen.
With this in mind, the reader might expect that Old Man Warner's time has finally come. After all, nobody else in the village has survived anywhere near as many lotteries as he. In a twist of events, however, Old Man Warner has another lucky escape when Tessie Hutchinson is announced as the winner. Jackson, therefore, suspends the normal laws of probability to create a twist ending.
A fine question. Given the size of the town (as indicated in the story), I'd have to say that it is the fact that Old Man Warner hasn't been chosen.
At one point, we're told the following:
"Seventy-seventh year I been in the lottery," Old Man Warner said as he went through the crowd. "Seventy-seventh time."
Given the limited number of names called, he should have been stoned to death a long time ago.
Now, emotionally, the chance that seems least likely is Mrs. Hutchinson first forgetting, then getting chosen.