ERA estimators are used to predict future ERA based on events that are most controlled by the pitcher. They remove the defense and luck of sequencing and only focus on these pitcher controlled events.
For example, Fielding Independent Pitching (FIP) is based on strikeouts, walks, hit by pitches, and home runs. If a pitcher continues at these current rates, one would expect his ERA to match his FIP in the future (or at least come close to matching it).
Skill-Interactive Earned Run Average (SIERA) is a more complicated ERA estimator that uses batted ball events to predict future ERA.
It makes sense to use these variables because they correlate so well with run prevention. For example, here is a graph showing how ERA correlates with strikeout rate for the last five full years, 2010-2014...
You can see that as the strikeout rate goes up, ERA goes down.
Now that new batted ball data is available, hard, medium, and soft hit balls, it is only a matter of time before a new ERA estimator is created. Just for fun, here is how ERA correlates with hard hit rate (for the same years 2010-2014).
It is not as strong of a correlation as strikeout rate is, but it is much stronger than line drive rate (2010-2014).
This means that when this new ERA estimator is created, it could be an even better ERA estimator than FIP and SIERA currently are.
Again, just for fun, I could use a regression equation in my program to generate a formula based on the variables I enter. It is not exactly structured the same way as the above formulas because those are based on run prevention. However, it would be interesting to see what the program creates and how accurate it is by the end of the season. I'm calling this HHERA, Hard Hit ERA.
I am using a 20 inning threshold to eliminate those with a small sample.
There are not many surprises here; most of the HHERA numbers are in-between the FIP and SIERA numbers. Kyle Lobstein's strikeout rate is a very low 9.8 percent, but his hard hit percentage is a below league average 22.6 percent which caused his HHERA to be below his FIP and SIERA. Alex Wilson has the lowest hard hit percentage on this list at 17.6 percent, which suggests that his ERA might not regress as much at first notice.