VORP

In baseball, value over replacement player (or VORP) is a statistic popularized by Keith Woolner that demonstrates how much a hitter contributes offensively or how much a pitcher contributes to his team in comparison to a fictitious "replacement player," who is an average fielder at his position and a below average hitter. A replacement player performs at "replacement level," which is the level of performance an average team can expect when trying to replace a player at minimal cost, also known as "freely available talent."

VORP's usefulness is in the fact that it measures contribution at the margin (as in marginal utility). Other statistics compare players to the league average, which is good for cross-era analysis (example: 90 runs created in 1915 are much better than 90 RC in 1996, because runs were more scarce in 1915). However, league-average comparisons break down when considering a player's total, composite contribution to a team. Baseball is a zero-sum game; in other words, one team can only win if another loses. A team wins by scoring more runs than its opponent.

It follows, then, that a contribution of any runs helps a team toward a win, no matter how small the contribution. However, the Major Leagues are highly competitive, and talent distribution in baseball does not resemble the traditional "bell curve" of a normal distribution; rather, the majority of players fall within the category of "below-average" or worse. (Since only the most talented baseball players make the Major Leagues, if all Americans' baseball talent was distributed on a bell curve then the Major Leagues would only see the uppermost edge of it, resulting in a "right-skewed" distribution.) Therefore, the so-called "average player" does not have a value of zero, like in Pete Palmer's Total Player Rating, but instead is a valued commodity. One alternative is to rank players using "counting stats"—simply their gross totals—but this is unacceptable as well, since it is likely that the contribution a marginal player makes, even if it does help a team win one game, is not enough to justify his presence in the Majors. This is where the concept of the replacement level enters the picture.

VORP is a cumulative stat or counting stat, not a projected stat. For example, if Bob Jones has a VORP of +25 runs after 81 games, he has contributed 25 more runs of offense to his team than the theoretical replacement player would have, over 81 games. As Bob Jones continues to play the rest of the season, his VORP will increase or decrease, depending upon his performance, and settle at a final figure, e.g., +50 runs, at the end of the season.

...
Wikipedia