Career ERA+

Cobby · 10-02-2019, 03:04 PM

Career ERA+ is not calculated correctly.

It appears that the current calculation is an IP-weighted average of each years' ERA+. But this doesn't work. If you simplify the ERA+ formula you find that IP ends up in the numerator instead of the denominator so it can't be right to weigh directly by IP. The other problem is those pesky 999 ERA plusses get weighed in. These are stand-ins for infinity and only serve to mess up the calculation.

The correct way to do this is to calculate ERA- instead. This will replace those pesky 999's with zeros. Then IP is in the denominator and you can do a weighted average by IP. Once you're done, invert the result to get back to career ERA+.

Let me know if you need further evidence or explanation.

10-02-2019, 03:04 PM	#1
Cobby Minors (Triple A) Join Date: May 2016 Posts: 252	Career ERA+ Career ERA+ is not calculated correctly. It appears that the current calculation is an IP-weighted average of each years' ERA+. But this doesn't work. If you simplify the ERA+ formula you find that IP ends up in the numerator instead of the denominator so it can't be right to weigh directly by IP. The other problem is those pesky 999 ERA plusses get weighed in. These are stand-ins for infinity and only serve to mess up the calculation. The correct way to do this is to calculate ERA- instead. This will replace those pesky 999's with zeros. Then IP is in the denominator and you can do a weighted average by IP. Once you're done, invert the result to get back to career ERA+. Let me know if you need further evidence or explanation.