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Baseball Hacks by Joseph Adler

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Measure Power with ISO

Use ISO to measure how well a player slugs the ball.

Isolated power (ISO) is a measurement of how well a player hits the ball, not just how often. A player wants to do several things during an at bat. First, he doesn’t want to get out. Second, he wants to give base runners the opportunity to score. Third, he wants to put himself in a position to score on a future at bat, or better yet, to score on the current at bat.

Two popular statistics—AVG and OBP—measure how well a player gets on base without making an out. But they don’t address how well a player helps base runners score, or how well he sets himself up to score a run from another player’s hit. A batter can help other players score by hitting the ball harder, allowing him to advance to second, third, or home base on the play (clearing the bases). A batter makes it easier to score a run himself by advancing to second, third, or home base.

Any bases a player reaches beyond first base on a hit are extra bases (sometimes called XB). You can define XB as:

	XB = TB - H

We want to measure how many extra bases a player gets: how many doubles, triples, and home runs. ISO is designed to measure the rate of extra bases hit by a batter.

Here is the formula for ISO:

	ISO = XB / AB = (TB – H ) / AB = SLG – AVG

ISO is the part of slugging average that’s independent of batting average. You can think of the relationship as:

	SLG = AVG + ISO

For many players, ISO measures slugging ability better than slugging average. A player ...

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