John Thorn

“If there is a man on third and one out, the team should score, on average, .897 runs. What does that mean? That 89.7 percent of the time, the man on third should score? No, not exactly: It means that the run-scoring potential is .897 as a function of there being a man on third and at least two additional batters in the half inning, barring a double play, pickoff, or failed steal attempt. Totaling the run potential of the man on third plus that of the two additional batters, who may get on base themselves, provides the .897. In the case of the first batter, let’s say that no one was on base—then the run potential for the team would be .249 (see the table for the intersection of one out and no one on base). Thus we see that in the situation this batter confronts, .249 of the team’s run value is attributable to the batter’s possibility of reaching base, bringing up not only the next batter but perhaps several more, depending upon the outcomes. This means that of the run value inherent in the situation “man on third, one out” (namely, .897), .249 resides with the batter(s) and .648 with the baserunner. In other words, a runner on third with one out will score, on average, 64.8 percent of the time.”
― John Thorn, The Hidden Game of Baseball: A Revolutionary Approach to Baseball and Its Statistics