RESEARCH: ERA/xERA outliers

Here at BaseballHQ.com, we like to embrace precision. At the same time, we understand the nature of any imprecision. A recent look at xERA, though, revealed some oddities. As part of the formulation process, we use a factor to adjust xERA such that league-wide, ERA and xERA agree. This makes sense, as there are likely factors beyond those in the xERA calculation that influence ERA.

However, if the xERA calculation was unbiased, we would expect the factor to be a random walk, sometimes above zero and sometimes below. In fact, it has gone in the same direction for the past 13 years.

So this doesn't look at all random. In fact, in each year for the past 13 years, the factor has been positive (i.e., calculated league xERA has been higher...

Almost!

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